Pro Ears has some high NRR muffs (non-electronic). Combine that with some foam plugs, and you'll get about the best NRR out there (31 and 33, IIRC).
If you want some decent electronic muffs, I use the Howard Leight Impact Sports. They only offer a NRR in the mid-20s, but when combined with good plugs, they work well. You can still easily hear conversations if you turn the muffs all the way up.
