I would suggest contacting the local
Lions Club. The Lions Club's community service mission is helping those with poor vision, and preventing blindness. They just might be able to link you to a low-cost service provider in your area.
If they are not of help, then you might want to consider going to Costco--if you are a member. I don't know for sure whether they can beat the prices at Wal-Mart, but it is possible that they can do so.