Formalin is just another name for paraformaldehyde in solution. However, formalin is just as difficult to obtain - Try asking for it in any high street chemist here in the UK...
Originally Posted by pdeeh
I've also seen acetone suggested as a substitute - The function of paraformaldehyde/formalin is to react with the sulphite in the developer and results in an increase in the PH (google for formaldehyde-sulphite clock reaction). Acetone also produces a clock reaction that raises the PH, but it does not get anywhere near the level needed for a lith developer. Another reason for not using acetone is it damages many plastics found in the darkroom.