Aussie,

DA50's come with stock 2.5 inch standoffs. Most places where you order the engine from will give you either the 2.5 inch or the 3 inch. Others will charge for the 3 inch standoffs.

You can use either. If you go with the 2.5 inch you will need a 3/4 inch block of wood behind each standoff to get the prop hub out where it needs to be. If you use the 3 inch standoffs, then you will need a 1/4 inch block of wood. In either case, it's important that the wood you use is exactly the same length or you will end up giving the engine some weird thrust anles

I ordered my second one right from DA at the last Toledo show. The first one was bought used on RCU. The standoff's are drilled and threaded for 4mm bolts (I think that's the size) on the engine side and 10/32 bolts on the firewall side. They are not drilled all the way through.

You also want to use nice big flat washers between the firewall and standoffs (car type fender washers work great) to help distrbute the load across the firewall better.

I know your "down under" so I cannot help too much with who can ship for you from the US but the first place I would look would be Troybuilt models (http://troybuiltmodels.com). They give you a choice of 2.5 or 3 inch standoffs