I got a 1991 firebird v8. It overheated on me so i checked the relay. well i found out that it went out. So i replaced it and the fan still wont turn on. What temp should the fan turn on? because i drove it till the temp was about 200 and checked it but no luck.

The fan should turn-on when the temperature is in the 220* range, assuming all is still set-up stock. Then it should turn-off when the temp gets down to the 195* range. I always say "range" because temperature-readings can vary from sensor-to-sensor, gage-to-gage, etc.

what's a good aftermarket
fan senor to buy if I want to hook that sensor up to the engine block?

Are you referring to a Coolant (Thermal) Fan Switch ? If so I have found three at Kragens. They have different turn on temperatures.
Part # TFS8 turns on at 220 off at 204
# TFS4 turns on at 227 off at 211
# TFS1 turns on at 238 off at 222
It dosnt say so on there web site but if you call them and tell them the part # for the Thermal fan switch they should be able to pull the actual temputare ranges up for you .Thats what I did and am running the 220 on switch. PS. The prices for the lower temp. switches are alot higher than the stock ones were used to.