BJS.com sells them slightly under face value, as does GamePointsNow.com for UK/EU cards. You might be able to negotiate a bulk price with either. Failing that you would need to go directly to the manufacturers.
Gift cards have among the lowest margin of any product you could possibly sell -- the extremely high inventory turnover and limited shelf space needed to showcase them (consider, you can put literally thousands of gift cards on a freestanding display just a few feet wide/tall, and store them in a spare corner of your stock room) tend to dictate that.
Frank