Why not just hire the interpreter? It will cost about $50 an hour for the seminar.
Or...
Find a local school that offers sign language classes. Offer to pay someone who took the class to come and be your interpreter. Okay, they may not be any good, but it will get you off the hook for a few bucks.
Dan
Call some of the larger churches in the area. A lot of the big churches have volunteers who sign that you could hire cheap to assist you. The plus side is you might also be able to pick up a client.