You need new rails with any after market seat for the Supra,
the main reason is the seatbelt buckle(what the belt clicks into) is attached to the actual seat so if you remove your seat you need some form of arm or piece of metal to bolt the buckle to.
I wouldnt advise a custom fabricated job due to safety, and instead I would get the Bride RO rails because they will be tested to a certain level of safety standard just incase your where in an accident, last thing you want is the seat belt not to work as per design
Also the seat bolts will not line up, after market to standard rails
Bride Rails for 2 you are talking roughly 300 euro shipped to Ireland, rails are about 80-120 euro rest is shipping
The cheapest I found was 300 euro for 2 shipped to Ireland via Yahoo Auctions, if you find anything cheaper let me know please