Look for a second hand gun in good condition.
You won't need a monster gun, something about 1.0 -1.3 metres long.
A wooden Undersee or Sea Hornet within this size range would be a good start.
Really depends how much money you have to spend, check out Ebay, hock shops, 2nd hand stores etc etc. Try and get someone who knows something about spearguns to go with you when you buy it.
Cheers and good luck.