You can look for stamp sellers on eBay, or look for the same/post a wanted in the Autograph Exchange/Sale section of this forum.
Alternatively, obtain paper currency from the return country through a bank, travel agency etc and enclose enough (concealing well) to cover return postage with your SAE. The smallest for UK is a £5 note I am not sure about AUS.
Look on the UK & AU Post Office websites to calculate the prices.

Could someone please tell me how to put return postage on an International package. I would like to send some requests to the UK along with Australia, but can't figure out how to purchase international postage. All the websites I have found will only allow you to purchase postage if you live in the country.

Thanks so much!