You can find them at Staples and probably other places, usually shrink-wrapped in packs of six or more - though I don't think I have seen nested sets of boxes where one fits inside another.
About mailing full manuscripts, only do so if they have been requested. Sending unrequested material makes a writer look like a hack. Always follow the submission guidelines (all agents and publishers have them) and send only the material requested. The first step is a query letter, sometimes with sample pages and/or a synopsis.
Secondly, once you've mailed it out, do you really want it back? It costs even more money and is a huge hassle for whomever is reading it. You don't want to leave an agent or publisher with the feeling that you'll be difficult to work with. Keep their dealings with you hassle free. And what's the point in getting it back? It will be dogeared with notes scribbled on it, and may bear a few coffee mug stains. Would you send that same copy to another agent/publisher? Just mail it (if specifically requested to) and give the receiver permission to toss it out when they are done with it. It makes you look more professional. The only reason to get a manuscript back is if you sent in an edit copy, and need to get the editors mark-ups for revision.
By the way, if the material was requested, most agents and publishers these days would prefer to receive manusripts by e-mail in .doc, pdf, or RTF format. Don't worry about which, they will tell you when they ask for the material.
Good Luck - Craven