Hi! Well done you for making the leap!!
I would ask as many people as you can for referals and then call people up and talk to them about your position and what you'd like to achieve. Most will happily come to you and as long as the field is reasonably flat and isn't too muddy / slippery, I'm sure they'll cope just fine. If you have no joy with referals, go to the BHS website, where there is a list of BHS approved instructors. They list phone no's and emails, so just give them a go.
Try the instructors out too... not all instructors will suit you / your horse, so don't be afraid of trying a few! As for rates, I don't know where you are, but here in Oxon you can pay anything from £15 to £50 for a one hour private lesson!
Good luck and have fun!