Best online shops to find details for an ALW Phantom costume?

There are no pre-made, mass produced Phantom costumes that do not look like crap. 你 will have to assemble yourself, 或者 commission a seamstress to get what 你 want.
I'm afraid your 问题 wasn't entirely clear, as well. Are 你 looking for a good ALW style costume (and then 你 need to specify stage 或者 movie), 或者 更多 Leroux? In the case of the stage style costume I know Phantom On A Budget offers tips to get the right look. Although, for the sequined cloak, you're going to either have to commission it 或者 get yourself a Bedazzler.
If you're wanting 更多 of a Leroux, then I would go on ebay to look for gentleman's styles from the late 1800s. The novel 说 that he dressed in evening attire (so very elegant and fashionable), that his suit was ill-fitting, and he wore a black full face mask. If 你 want something fancy, Venetian masks can be found. Papier mache would be period appropriate, 或者 你 could do what I did and make your own (mine looks like s**t, but it was a first attempt and it's still kinda cool
Goodwill and other 秒 hand 或者 retro stores will also be your friends.
Other than that, throw me some specifics on what 你 want and I can give 你 更多 detail.