Just go to Costco and get a standard sized Kirkland for a decent price. Has worked in my 91S for years now.
Some people insist on using the OEM recommended Interstate. Those can be found, but I've never had much luck with them. The one in my LS is very weak, always has been, and the previous one in the S was short lived. The Kirkland has worked much better. The LS will soon get one as well.
Do keep in mind that if you wish to use the Interstate, there are two almost identical models, one good (I guess), and one crappy, according to a recent Consumer Reports magazine. Make sure you get the better one.
89 Milano (wife's daily driver since 1989)
91 164S (my daily driver since 1994)
94 164LS (~Q) (trip Alfa since 2000)
72 Morgan 27 (water time since 1976)
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6