i used to get my clear envelopes at this site.
http://tinyurl.com/zgxcoc3 $9.99 for 500. they come in five 100-pks. you could split an order 5 ways. if anyone decides to order, i will buy a pack from you.
you write the address directly on the postcard with a pigma pen, slip it in the clear envelope, and put the stamps on the envelope so the post office can hand cancel it.