i love tuners the supras, rx7's, and the 300gt's but their hard as hell to find.
Gotta look in a lot of places if you want to find them in good condition: craigslist, newspapers, autotrader type sites, car forums (local forums and car specific forums alike), and put WTB ad on craigslist might help but beware of scammers. Price for a good condition RX7 (what year? 1979-1995!) anywhere from $2000-$8000, to $5000-$10000 for a 3000gt VR4, or from (mk3 supra or mk4 supra?) $3000-$6000 for a decent MK3 Supra, or anywhere from $12,000-$25,000+ for a decent mk4 Supra.
as they guy said you can search craigslist ebay autotrader yahoo motors, and 3000gt are easly available much more that supras or rx7's. rx7 you can find alot of them also, not so much as supras. As for the price 3000gts are cheap the front wheel drive ones but the 3000gt VR4 are expensive as hell aroung 15k-20k, rx7 depends on the year, 93' and up almost never go lower than 10k alot are 16k, and the supras depends on the year 94' and up are crazy ass expensive 20k and up you can even find ones for 70k but most and from 30k to 50k