Most any 'seed starting supplies' supplier carries them. Several source recommendations included in this discussion from further down the page. Price will all depend on how many you order - bulk orders being the cheapest of course.

Just a few:

http://www.growerssupply.com/farm/supplies/cat1a;gs_seed_starting.html

http://www.greenhousemegastore.com/category/seed-starting

http://www.gardenharvestsupply.com/ProductCart/pc/-c672.htm

http://www.novoselenterprises.com/

http://www.dirtworks.net/Seed-Trays.html

Dave

Here is a link that might be useful: where to buy trays etc.

thx...Looking at the sources for best value....Jack