Finding inexpensive lens caps can be a hassle. Local prices are usually high (I don't blame them) and eBay sources usually list one or two and by the time you've brought enough caps the shipping fees are killing the deal. After quite a bit of searching, I found Deal Extreme. Their prices on lens caps are quite low and the shipping is free. They do come via Hong Kong though so one must be patient to wait on the postal system.
I've brought both the cheapest lens caps offered at Deal Extreme and the slightly more expensive models. Comparing the two, I prefer the build and handling of the slightly cheaper model.
Examples:
DealExtreme: $2.28 49mm Digital Camera Lens Cover
DealExtreme: $3.37 49mm Digital Camera Lens Cover
The good news is the prices stay reasonable for 72mm ($3.87 but only a model like the more expensive 49mm is offered -- I wish they had the other one) and there are quantity discounts. I was able to outfit all my lenses without spending a fortune. Finally, I believe the free shipping is world wide.
Does anyone else have any other sites or sources?