Climatically Cambodia probably comes closest to the 'Top End' of the Northern Territory or far north Queensland.

There are many attractive flowering trees and shrubs native to these regions, e.g. Eucalypts such as Eucalyptus miniata, E. phoenicea and Corymbia ptychocarpa, and many species of Acacia, Brachychiton, Calytrix, Grevillea, Xanthostemon, among others.

The only seed supplier I could find listed for the Top End was:

Top End Seeds
2 Leung Crt
Milner 0810 NT
ph (08) 8985 6885

It's worth noting, though, that many of the rainforest and vine-scrub trees from Autralia's far north are related to and quite similar to those of South-east Asia. These include genera such as Ficus, Terminalia, Gardenia and so on -- a great many in fact -- as opposed to the more distinctly Australian groups I have mentioned above. I suspect there is a greater wealth of potential ornamentals from these rainforest groups among the native Cambodium species than the Australian ones. I have spent some time in Thailand (whose flora coincides with Cambodia's to a large degree) and have found that plant enthusiasts there are beginning to look to their local natives at last. I expect that Cambodia lags behind a fair way. I found a small nursery on the outskirts of Prachinburi, the owner of which was actively collecting and propagating some beautiful local natives.