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