well it really depends what you're after...
besides cranbourne music, there is allan's and for acoustics there is the acoustic centre...
there is a famous 2nd hand music shop near carlton but its name escapes me for now.... its quite an overpriced place, but you can get some old amps there for cheap...
ebay there would unearth you some gems
but for a lot of stuff tho, joel68 is right... prices there are high, but i would like to add that the range of stuff that you could get there is probably wider than singapore too...
cash converters is ok--- but prices arent that great... you should check the local ads or ebay (although im not sure about the whole paypal thing now)
hope that helps