Mike --
There are many, many on-line stores for wooden screen and entry doors. Following are only a few that may sell these types of doors taht you're looking for as well as other architectural woodwork and related trim. There are about four times as many that don't have their own websites but sell through direct mail order. I'd suggest you look in your local Borders or Barnes and Noble and look on their magazine racks for something called the Old-House Journal Restoration Directory -- The National Catalog of Suppliers. This Directory is a compendium of all the suppliers you'll ever need.
BTW, I've looked into these myself. Have wanted to buy mail order or on-line but the shipping rates are a killer. Because of the size of the door, many of these companies can't ship UPS, so they are shipped through freight handlers. These guys are very expensive.
Hope all this helps.
http://www.gingerbreadman.com/
http://www.madriverwoodworks.com/
http://www.millworksupply.com/
http://www.oldhouseparts.com/
<A target="_blank" HREF=http://www.woodstone.com/>http://www.woodstone.com/</A>
Regards,
Bob Ancar
Cambridge, NY