At the Lomographic Society in Austria you can buy cardboard cameras as well as more long lived types, such as Zero Image cameras, kits for building your own paper cameras and also Pinhole Blender, which is a lovely camera, even though I find it a little difficult to control. Lomographic Society is also the place to buy the Diana+ or the Holga camera with a built-in pinhole feature. lomography.com/shop
Pinhole Resource in the US also has a vast range of cameras, anything from DSLR camera pinhole body caps to large format cameras and anamorphcic cameras that take 120 format film. pinholeresource.com/shop
Also check out the beautiful handcrafted ONDU cameras!
Zero Image in Hong Kong sell all their beautiful wooden cameras in their online shop. Models range from 135-film types to large format, De Luxe models with built-in bubble levels etc. zeroimage.com
Freestyle Photographic Supplies sell paint bucket cameras, wooden camera kits, large format cameras and other goodies. freestylephoto.biz
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Please note that I did not try shopping from all these online shops myself and I do not guarantee the reliability of any of them. But I think they are OK!