I won't say that it is a common error, but it certainly is not uncommon - an experienced technician, paying attention to what he's doing, shouldn't make it, an inexperienced owner, doing a DIY rebuild might - some head gaskets are marked "TOP", some aren't - but, an experienced assembler should know that he/she has to check to make sure that the various holes and passages all line up correctly.
Part availability - in my neck of the woods, I probably could get parts, but I suspect it would be less expensive to buy a complete used engine - if you're in the US, Suzuki's departure from the automotive market has made it difficult to find parts, but, there are a number of off road specialty shops who disassemble and should have parts.
Low Range Off Road are showing remanufactured heads on their webpage - give them a call.
