Mainly on ebay… The most current model is the soviet (!) LOMO UPB-1A that can handle one or two layers of 50 ft of S8 or 16 mm film, or even one time 50 ft of 35 mm. Theoretically, it comes with a very practical light-tight tank.

Be careful when you buy, some other LOMO models only take 25 ft or 33 ft by layer !