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