The humble folder could be just that - humble and inexpensive - or it could be a superb professional picture taking device such as the Voigtlander Bessa 2 with the Lanthar apochromatic lens. I always desired but never have owned one of those, the holy grail of folding cameras. Folders, cameras with small bellows that fold down flat when not in use and, a huge plus, will therefore slip easily into a jacket pocket. They are usually roll film formats, anything up to 6x9cm, and the best of them will produce stunning medium format quality. Films are generally 120 size, still available, but most formats can be used with respooled film. For example, 620 film is the same dimensions as 120 but just on a thinner spindle, so a roll of 120 can happily be respooled. There are some very unusual formats as well and some may be a challenge, but I'm sure there will be somebody, somewhere, who can supply the necessary film stock.
It's important that when open a folder will click firmly into position, leaving the lens panel accurately square with the film plane. It's also important to, wherever possible, find the best lens options as the results will vary depending on the spec level. Most cameras out there will be low end versions, but there will be a few stunning gems to be found yet. Here's a few pictures of folding cameras that will fit the pocket.
Kodak Vest Pocket Model B
Houghton Folding Klito Junior
Kodak Folding Pocket Brownie Model B
Soho Pilot
Kershaw King Penguin
Soho Cadet
Butcher Carbine
Kodak No 3 Special, 1902
Folding Pocket Kodak No 3, 1909
Nixette, 1955
Solida III
Kodak 620 Junior
Baldaxette II, 1936