Hmm, it really depends on what is your budget. Options in Whiteflash will have some of the best cut quality, but they comes at a premium. You can also check out Brian Gavin Diamonds. Their top of the line collection is Brian Gavin Black, something like this - https://www.briangavindiamonds.com/collecti...?a_aid=myengexp (affiliate link).

One interesting to take note is that AGS has merged with GIA and they no longer appear as a single entity. The stone I listed above is actually graded by GIA.

However, Brian Gavin does not offer free shipping. You will have to pay more or less USD 50+ for shipping to Malaysia.

It is always best to start with a budget that you would like to spend on the ring. For your budget, at this moment, my preferred choice will be the Brian Gavin Black 0.45ct diamond for its size.

If I want to go on a more budget option, I will probably pick this: https://www.whiteflash.com/loose-diamonds/0...id=myengringexp (affiliate link). WhiteFlash does provide free shipping to Malaysia.

Again, do note that diamonds from WhiteFlash or Brian Gavin Diamonds do come with a premium. For similar 4Cs, typical GIA diamonds that have the same 4Cs can be cheaper.

Since you mentioned ZCOVA, you can actually engaged them and let them know what is your budget and 4Cs preference, they are able to assist you further. Then, you can start to evaluate your options and see what suits you best.

As for ring setting, a lot of jewelers are able to do it, but their price may be higher if you only do the setting alone.

I do provide ring setting as well for Klang Valley under my own registered enterprise. My plain ring setting starts from RM 1600.