Okay, we'll get started by listing the sources and models currently available or recently on the market (ebay):
For 1st Gens, there're two games in town. The high end is represented by Exact Detail (Lane Collectibles). They offer a super-finely detailed (and expensive '67 and '68. Street price on them are around $100 each. At the other end of the price scale, Yat Ming offers a pretty decent '69 TA in the $20-$30 range. Of course we all know that 1969 Trans Am's only came in one color, which gives a wink and a nod to the silver and orange versions offered by Yat Ming!
(Note: since these images were produced, other color variations and models may have been released. My collection is a little more current than my documentation.)
For 2nd Gens, AMT/Ertl is the main player with models representing 1970-1/2, 1972, '73 and 1977. The '77's were released as both a Trans Am model and a Formula. Of particular note were the Bandit variants, in both black and black chrome. There is also a ModelShop Bandit that you get to assemble. Yat Ming also offers a very good 1979 model that will suffice for all '79-'81 owners out there. They are relatively easy to take apart and strip/prime/paint to match your 1:1 scale ride.
Third Gen fans have so far been pretty much out of luck, with the exception of the Ertl/Joyride series that featured both KITT and KARR from the Knigh Rider TV series. They are out of production now and examples are going for nearly $100 on the secondary market! But there is light at the end of the tunnel - Greenlight Collectibles
http://www.greenlighttoys.com is soon releasing several versions of the 1989 TA, including a GTA and the Turbo Pace Car.
Fourth Gens are available through Ertl ('94 Anniversary Edition and several 1996 variants) and Yat Ming for the '98 thru '02 versions, including the 30th 'Vert and CE 'Vert. Unfortunately, no coupe versions of the '98-'02 model exist, although I've been toying with the idea of combining the Yat Ming and Ertl models to see if they might fit.
Please don't ask if I'll sell any of the ones I have or otherwise part with them. I'm a collector on a budget and, in most cases, I did not acquire any duplicates. My best advice is to scour ebay and other online auctions or spend time visiting hobby shops and swap meets. Very often, you can get lucky! As with smaller (1:43 and 1:64) diecast, there were occasional variations involving wheels or other details, like opening trunks, that make collecting these a rewarding challenge.