Try Foto Hobby in Silom Center, third floor. He usually has some used Rollei gear. Very friendly place, and a good place to buy film as well.
Edit: AV Camera in New Road (to the left of Robinson) might be worth a try, but that's a longer shot...
I think he said he tried Foto Hobby already :)
But AV is a good suggestion and I was even thinking down around that area in China Town (Pirom Plaza) where they sell a lot of older film equipment they might have something.