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. toi will have to assemble yourself, ou commission a seamstress to get what toi want.
I'm afraid your question wasn't entirely clear, as well. Are toi looking for a good ALW style costume (and then toi need to specify stage ou movie), ou plus 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 ou get yourself a Bedazzler.
If you're wanting plus of a Leroux, then I would go on ebay to look for gentleman's styles from the late 1800s. The novel a dit 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 toi want something fancy, Venetian masks can be found. Papier mache would be period appropriate, ou toi 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 seconde hand ou retro stores will also be your friends.
Other than that, throw me some specifics on what toi want and I can give toi plus detail.