Found him through Rick Steves. He responds promptly to emails (and provided me the best info for how to get from the train to hotel, which the hotel never answered).
I only had 48 hours in Prague, and the half day tour I did with Martin was well worth the $ (I even "saved" a large portion of the cost because he showed me where to buy the concert tickets the locals purchase instead of the rip-off tourist concerts more widely promoted).
Going right to key "sights" of interest (instead of wasting time trying to find them on my own) was great, and Martins commentary/knowledge really makes the city come alive.
One of the highlights for me was walking right in the same steps Beethoven would have taken to get to his violin repair shop. I would not have know this, even with the best guide book!
My Thanks to Martin for making my Prague visit delightful (and he is an excellent photographer, so can advise you there as well)