Ke... I looked on the web at a few different places but ended up going through Mayawalk to do the Mountain Pine Ridge tour (they charged us $45 instead of $35 since there were only 3 of us signed up, but still well worth it). I did Xunantunich sans tour operator, with a couple I'd met earlier in my trip... we hired a taxi driver to take us there and wait while we got a guided tour. All told, that cost us less than $15 US each (taxi and tour guide). It seemed like a lot of the operators in the area charge very similar prices so you can't really go wrong whoever you end up going with.