I'm going to go against the flow --- for your first actual costume, by all means *do* go to wholesale online stores. Emna Ai seems like it would be pretty easy to find there.
A commissioner is someone you contact with reference pictures asking to make that costume for you, they will give a quote of the amount they'll need to make it, you'll need to give them details and your measurements, etc, etc.
Short: It's someone who tailor-makes a costume for a someone in exchange for money.
I wouldn't recommend it for your *first cosplay ever*.