I used this website to rent an additional suitcase for my 8 day trip from New York to London www.theluggagerentalcompany.com
Their prices are fair. Although they do charge the deposit upfront. The bag was delivered the date we requested and it seemed to be brand new. Perhaps I was lucky? When I was back, I FedEx'd the bag in using the included postage and I received my deposit 5 days later.
Overall a recommended service if you need additional luggage for a single trip and don't want to purchase to own.