Yes, adaptor plugs are readily available in Bangkok. They're pretty cheap - a couple of hundred baht max.
I've bought them in Robinsons Department Store, Silom but I'm sure you'd get them in other Robinsons stores as well as Central Department stores.
Maybe Watsons Pharmacies sell them (?)

Your hotel should have adaptor plugs and will lend you one while staying there.

If you haven't left Australia yet, you can buy them at travel sections of department stores and luggage /travelgoods stores.
NRMA/RACV/RACQ shops sell them too.