Hi there, fellow 1300 Junior owner! I have one. The brake booster is actually a very common part shared with many of the early ATE brake equipped cars. It is available from some f the parts vendors which can be found in the resources section at
www.alfacentro.com.
By the way, Jim (Papajam) is a really knowledgeable and helpful Alfa mechanic, as are several others on here. Welcome to the BB!
I actually had the same problem and have a spare booster, which I will install eventually. For now, what I did was to unplug the booster and run with unassisted brakes. They work just fine! And it actually seems to give me better pedal feel. I wouldn't do that in a 'hanging-pedal' car, but in the Junior, it works great!
So are you restoring yours? Where are you located? Do you have a Canadian model (with gauges in English and with MPH on the speedo), or do you have a Euro-version?
I'll attach some pics of mine in another post.
Cheers,