Just a few things to keep in mind:
If you are going to have costumes that you have NOT purchased as stock, but have there on consignment, there are certains ways that you must legally treat other people's wares. For example, if you decide to have a sale, and you say 15% off everything in stock, you not only are then legally obligated to sell that person's item at the discount to the customer, but you must also make up the difference out of your own pocket when you go to pay the vendor. And if anything got stolen, YOU would have to pay the person for it.
I think it's a great idea to have that kind of stuff, but speak with a lawyer about a consignment contract, or perhaps consider some sort of "rent a rack" program instead of straight consignment. You have to cover your butt, legally
All that said, if you're going to be in the portland area, I am a plush artist, and i would totally bring custom plushies in to help fill up your shelf space
In addition to the anime club idea, I think it would be cool to have some sort of weekly game thing..and you could attract different fans each week. Something like come learn the card game in fruits basket (i think it's called rich man, poor man)- then set up a couple of tables, get the rules, cards, push all the fruits baskets merchandise to the front, and make some cash
They play games in lots of series, just pick one, and totally pimp it to that series' fans.