Keep an eye on Playbill for job postings. I know I've seen postings from Araca, Creative Goods, Marquee Merchandise, and Max Merchandising fairly regularly. You'll probably start seeing some of those again soon as the spring rush of openings happens and they have to beef up their staffs. I'm not sure how Araca pays, but I believe that Creative Goods pays an hourly rate, and Marquee does a flat rate plus sales commission.
Typically you're looking at a call time from 60 to 90 minutes before the show starts, and that would be based on where the merch booths are located. If it's somewhere like the Beaumont where patrons are able to get to the booth before the house opens then it would most likely be 90 minutes, whereas a space like Studio 54 where the booth is normally in that inner lobby where they can't start selling until the house opens then you'd be looking at a 60 minute call time. I would assume that you're probably looking at having to be there until 30 minutes or so after the show ends between walk out sales and having to close down for the night.