Honestly, unless you're completely hamfisted with basic tools, it's probably simpler to make a pinhole camera than to buy one. If you want to use 120 film, you start by prowling second hand stores and eBay until you get a 6x6 or 6x9 folder with a bad bellows, cheap lens, etc. -- you'll use it for the film transport, which will save you a dozen hours or more building a film transport, frame mask, etc. and getting it all light tight. Use appropriate technology to remove the lens, shutter, bed/door, and bellows, leaving the film transport and frame mask naked, then fabricate a light tight structure on the front from thin wood (painted black -- thin wood isn't light tight on its own), black-core foamcore, black-core matt board, sheet metal, or sheet plastic (again, painted black for light sealing), depending on your preferences and comfort zone. Make the pinhole from the aluminum of a soft drink can or heavy duty disposable pie tin, brass shim stock, etc., and if needed fabricate a simple shutter from the same material as the camera body.

Not much more work than assembling a kit, and you can pick your own focal length, get a roll film transport, etc.

Or, if you have a folding camera with a good bellows, you can simply unscrew the front and rear lens glass from the shutter and replace it with a pinhole, as I've demonstrated here (my example was the easily removed, bayonet mounted shutter from a 1920s vintage plate camera, but the same process could be done as easily with the shutter still mounted in a roll film folder, or even a TLR). This gets you a "normal" focal length, so no super-wide shots, but it also lets you use a shutter that's reasonably accurate on short times, which can be helpful if you shoot faster film in bright sun; ISO 400 with an f/250 pinhole requires about 1/4 second in full sun, which is a little short to time with black tape over the hole...