Sometimes you'll find kits cheaper if you search for "Cressida" instead of Supra... There are a couple different gaskets on the intake but that's all, EVERYTHING else is the same.
I've had good luck order OEM type parts from an eBay vendor named Mizumo Auto Parts. I don't see any full kits in their listings right now but you could contact them to see what kind of deal they'd make you. If you were to buy parts from their current listings you should be able to piece a full kit together for about the same price.
http://cgi.ebay.com/Toyota-3-0L-DOHC...item3cb2a23c8c
http://cgi.ebay.com/89-92-3-0-L-Toyo...item53de461fbc
http://cgi.ebay.com/Toyota-Supra-Tur...item3cb29be5a0