Technobots do both Spur gears and sprockets+ chains for what looks like a reasonable price. The spur gears do have hubs on which may or may not be a problem. I can't vouch for their quality as I've never bought them myself, but I'd imagine they'd be good. Plus Technobots usually ship quickly which is probably a bonus at the moment!
http://www.technobotsonline.com/mod-...pur-gears.html