I have been making inquiries, and will share what I found out so far, so it may benefit anyone else wanting this information in Cambodia.
The best place for thalassemia testing in Cambodia appears to be:
Institut Pasteur du Cambodge
No. 5 Monivong Boulevard
Phnom Penh
Cambodia
Tel: (855)23 428 561
Mobile: (855)12 812 003
E-Mail: accueil@pasteur-kh.org
From a brief phone discussion, it seems they do blood tests, but not DNA tests. I am led to believe they can send samples to France for DNA testing.
In Cambodia, very few people have heard of thalassemia. People have poor health, and they don't know why.
In Malaysia it is common knowledge. I read somewhere there is talk of compulsory testing for everyone before they get married.