I buy nearly all my South East Asian airline tickets on Expedia. I know using a travel agent is not a popular choice for some but it’s easy, plenty of choice on those route that have that, usually no more expensive, pay in pounds, get some cashback and they are reliable to deal with. Some of the airlines own websites seem difficult to use.

However, some airlines are not on Expedia so you do have to search a bit, for example, on ITA Matrix.