a quick search of Google turns up three spots you should check out in your area:
AAA Alternators & Starters (206) 214-0699
Allstar Alternators & Starters (425) 867-1159
A-1 Alternators & Starters (425) 451-3446
now, if you wanna know which of these may or may not be the choice for you by asking folks with experience....you should be asking on a USA forum, not a UK forum. try
www.flu.org or
www.fiatspider.com The folks that frequent those forums are better able to deal with this type of inquiry.
I don't know any reason why an un-airconditioned 1980 Fiat Spider can't reliably use a rebuilt (not reconditioned) 55 amp alternator. I get all the juice i need from mine, and always have. The higher output alternator was introduced for the air conditioner unit, not because the 55 amp was inadequate.
Of course, if you wanna discuss replacement rather than rebuilding, i have several alternatives for you as well....but that is not what you asked. The guy at Advanced Auto only knows what his computer says, nothing more. He is no expert, just capable of reading a computer screen.