I've never located one, but it is just so easy and quick to get them out of Europe that I haven't had much interest in trying.
The regular sources (OKP, Heinbrand, etc) stock them. I got a couple from TST Rettberg. but they seem to be at least temporarily not showing up to work. OKP gets me stuff via Fedex in less than a week, and the freight cost always seems reasonable.
The sets that seem to be out there appear to be a bit more appropriate for the 1900, but the key gaskets work OK. I wasn't happy with the rear plate gasket, and there are a few that aren't included at all. Still, it's the best you can do.
Are you looking only for a complete gasket set, or just a few select items?
I am becoming aware and have worked with some of these sources, just hoping someone had one in the states.
I really need what's needed for a rear crank seal replacement(oh fun!) but am planning a complete rebuild down the road. I believe a oil pan gasket and a rear engine cover gasket is needed for this job. I don't know what else, but I don't like surprises.
The rear oil seal is typically not included in the full gasket set, nor is the front. I've bought these from OKP, but imagine one could dig around on the BB and find the metric size for this and order it from NAPA or similar.
The original rear plate gasket is a relatively hard and thickish piece with fingers that extend outward to the extremities to match the bolt pattern for the rear plate. The one that comes in the modern gasket sets are thinner and don't have the upper fingers. One could just ignore this and hope the pinch doesn't create problems in the aluminum rear plate, or you could cut out gasket material from stock in the same thickness and eliminate the worries.
You'll need a lower pan gasket as well, of course.
All of these could be made from stock gasket material except the oil seal, and like I said, I suspect you should be able to hunt that down via domestic sources.
Alfa Seals can be ordered from American sources by using their dimensions in metric. Us bearing houses sell seals in metric measurements. I have purchased rear main and front cover seals from US parts houses for years: DIMENSIONS as follows: "OIL SEAL RING for REAR cover: 100 x 125 x 12" This is same seal in Alfa parts book numbered 2340.34485D (incidentally the same part number for the 1900 usage). "OIL SEAL RING for FRONT timing system cover: 55 x 72 x 10" This is the same seal in Alfa parts book numbered 2340.34435D (also 1900 usage part number).
So you can always go to a good old Yankee style parts house and order via the dimensions too. The first number is the inside diameter, the second the outside, and the third is the thickness. Might now actually get seals made in Japan or China instead of Germany, France, Italy or USA, but all fit and work. Incidentally, I might add that except for the shouldered bearings in the split case transmission that seem to be specifically Alfa, I get my bearing the same way. Just use the metric dimensions.
As for the specific two liter gasket sets I learned over the years that engine gasket kits from Alfa used to have all the seals, and decent head gaskets besides, whereas after market kits not always (and those from Beck also had lousy head gaskets). I used to buy head gaskets six at a time from Alfa. My spider was my daily driver from 1965 to 1990. I suspect AFRA still has them. If not, order a 1900 head gasket which comes with a copper sheeting on both sides and is reusable.
Finally, I long ago carefully traced out on gasket material the rear cover gasket with the fingers, using as my pattern the pieces of one I was careful about when removing. I believe the factory gasket for that part too stiff and prefer the gasket material I can buy in rolls. I also cut my own one piece pan gaskets from the rolls instead of using those four piece cork sets provided in gasket sets.
Sorry, I should have commented that two liter is one of the few Alfa engines on which it is easy (relatively) to replace the rear main seal. You are correct that the pan has to come off as well as the rear plate. But one does not have to remove the head or even disturb the timing. I don't take the engine out the car, but have to undo the motor mounts after removing the transmission and driveline so I can raise the engine enough (have to get some exhaust slack too) to get the pan off. Clutch comes off flywheel (with disk), then flywheel, then rear plate with seal. Clear the plate and replace seal, fit new gaskets on reassembly (including the pan gasket). But one of the joys of two liter is that you can hang the clutch pressure plate on input shaft and push disk on splines of transmission instead of putting it on flywheel. After you put the transmission bellhousing on the engine the clutch is accessable through the inspection plate and you can bolt the pressure plate to the rear of the flywheel then.
In taking all that apart you will usually find something else you will think you need to fix. And put all the bolts in the back plate when you put it back on the engine (from experience -- I left one out once and had to do it twice). It's no fun to do this over and over, so take care of throw out bearings if they are noisy, or get a new clutch disk if you are down to rivets. Resurface the flywheel is the grooves are deep and disturbing. I happen to have the factory tool to reset the clutch springs on the pressure plate, (but I've never found one that needed to be completely torn down).