As far as non-electronic muffs go, anything with a high NRR will do the job. The higher the better.
These would be good: http://www.walmart.com/catalog/produ...#ProductDetail
No mater what the muffs I have are rated for, I always double up at indoor ranges, plugs and muffs. You never know what someone else is going to be shooting next to you. Someone brings in their hand cannon and plugs or muffs by themselves just aren't enough.