There are a number of great vendors who keep us supplied, most are generalists and offerings overlap, but each also seems to have strength in one particular area over another.
The ones that are regularly mentioned (many have strong presence here on the forum) are:
I will only comment on those that I have PERSONALLY BOUGHT FROM.
Auto Ricambi US (in Texas) Highly recommended. Great Service.
Allison's Automotive (California, great performance parts specialist, runs this forum.)
Vicks Automotive (located in Texas, both general and performance parts supplier)
"SpiderParts" Private enthusiast, here on the forum. John is a fantastic, helpful fellow, located in Baltimore.
Mr. Fiat (HAS YOUR SEAT BOTTOM)
http://mrfiat.com/italian/fiat/fiat-124/interior.html
IAP (Fiat section gone now, bought by Auto Ricambi)
Daniel Stern Lighting (automotive lighting specialist)
I know that I have missed some, sorry, There is a forum thread called "vendors who have treated me right" or "well" or something like that, where a long discussion has been held, and many comments placed. It can be found with a simple search. The concept of vendor bias (both positive and negative) has, quite unfortunately, become a sore topic and has led some folks to actually leave recently. It is a shame.
But anyway, your seat bottom is available!
Pete