As other people have said, steps in town are a good place to start, and there are loads to choose from. The wall in Castle Park or the concrete bank at the corner of Baldwin Street are good because you can build up from small to fairly sizeable. It's a bit embarassing riding your MTB in town and getting shown up by the kidders, but you'll rebuild your confidence really quickly
If you want "natural" drops out on the trails, the couple we built at the end of 50 Acre are good – once that bit of the trail is open again! (should be happening soon) Or there are a couple of small ones on the first bit of the TT in Ashton Court, along from the Gatehouse, which should be OK for a practice at quieter times.