I take mine to a local shop. I pay anywhere from $5-8 per 8x12 (depending on how many I get done at one time.) Most photo labs will actually do enlargements. Some send them away and some do them right there in their lab. Just ask your local shop. Any shop that does enlargements should offer bulk pricing if you need a lot done.
When looking for a shop asks if they do them manually in a darkroom. Some still do and you can definitely notice a difference in the quality.