my transmission has gone bad

hi all..im pretty new to this place..i have a 3.8L 2000 mustang and i guess my tranny is gone it doesnt transmit smoothly and i see a lot of variations in the rpm..typically if i go more than 55 mph..it doesnt even switch to the next gear..it stays in the same one n my rpm reads more than 3000..i have stopped going on highways..i just take normal service roads..i dont know how it happend..it has run just 85k miles..i stay in hartford ct..could u please give some suggestions on how to get it fixed. I dont want to go for a new tranny coz that would cost me like 3 grand..any place to get a used tranny for cheap? like do you know any junk yards or anything like that which can be trusted?
i dont want to sell my car either..i love it..