Maybe not bus models exactly, but while I was out today I thought about other bus model/toys that are available and aren't too costly. I'm always on the lookout myself to add to my collection, but there are a few stores that sell bus model/toys around Canberra.
Little Sprout in Phillip sell a range of buses standard, coach and artic, they also sell trams in the same style and will cost around $8-$30. Sometimes they don't have any though, I'll let you know if and when they do if you're interested as I'm near there often.
Big W and Toys R Us are selling the larger
articulated buses made by either Dickie Toys or Fast Lane. I'm not sure if Big W have put the price up since I last looked of $16, but Toys R Us are now selling them for $19. The current ones that are available are in the red and white variants (blue and yellow were the previous variant, and an older model style before that in blue, yellow and red with green only by Dickie Toys shortly before the new model started production).
Matchbox sometimes has buses too albeit very rarely, the last variant being red double deckers a couple of years ago. I managed to buy about 14 of them myself after finding a couple left over after them not being in production for some time a David Jones (as shops like that don't sell that sort of thing quicker than other stores - sometimes things aren't bought or are left in storage for years).