BEST way is to befriend fellow collectors in those countries and do a periodic stamp exchange. ie every few month send off a bunch of Aussie stamps to them and they send US/UK stamps to you. Or send them some $ (or the equivalent value signed item for some stamps). But you need to keep up to date of the international postage rates for that country as cost continue to rise.

You need some form of return postage if you want your item back/ a photo sent.

AUS stamps won't work on a UK or USA SASE. As zzublue has said, if you can't get these, then enclose IRC's.

If you are not going to put stamps on/in your SASE, then add several IRCs and it will be okay. But the best way is to put stamps on your SAE, so that the celebrity doesn't need to make time for exchanging IRCs for stamps at his/her local post office.

Is it ok not to put UK or USA stamps inside the SASE when sending from AUS?