I was in Budapest at the end of last year, and managed to find a little second-hand shop that was like heaven. Walls covered in boxes of film, and Foma & Forte papers stacked up to waist height - it was great! (Oh, and a plentiful supply of very cheap Zorkis/Feds/etc.)
Of course, I can't remember an address; but I'm pretty sure it was somewhere along the walk from Blaha Lujza tér to Erzsébet Hid (Elizabeth Bridge) - I think along Szabad Satjó út or Kossuth Lajos utca; it was opposite a small square with what looked like a modern brick/concrete bus station on one side (I don't think it was a bus station, but it looked like one!) and a statue of a chap wearing a cape on a large plinth in the middle of the square/garden.
Hopefully that might be enough for someone who knows the area properly to identify the location!
Anyway, enjoy Budapest - it's an absolutely beautiful place, and the locals are very friendly with it. The language isn't as hard as it first appears either! Try a trip to the Statue Park on the edge of town where all the Soviet-era statues have been relocated to - my avatar is a photo of one of them

. (If you take that route by public transport, it'd be worth at least having a phrasebook with you as its not really a tourist route and you need to change buses in the middle of nowhere - when I did it I nearly missed the stop entirely; fortunately some of the other people on the bus had worked out where we were going and made sure we got there!)