Fix stamps or let postal service fix.
You should be affixing your own stamps in most cases. Unless you have stamps and the desire to coordinate stamp designs and cancels, I suggest you use something light-colored and fairly neutral, i.e., the current US flags definitive/regular issue stamp.
May I ask, exactly what are sending away for?
• US only?
• first day covers?
• normal city and town cancelled covers?
• special event covers, with or without special cancels, like for stamp shows?
• all of the above?
• something else?
That said what type of envelopes should I use(type and size)? Where can I purchase them at?
Already answered here:http://goscf.com/t/83109
You need to add something inside the envelope to get a good cancel. A piece cut from a manila folder works well or use something of that weight and flexibility. Tuck in the envelope flap rather than sealing the envelope.
Can I print my own graphic/cachet to these type of envelopes?
You sure can. Make sure the ink used doesn't repel whatever cancel ink might be used by the post office. Otherwise, a smear or smudge is guaranteed. Depending on what kind of covers you are after, you might try sending a cacheted cover to yourself first, as a test.
Address it in soft pencil, like an H or HB; that can be easily erased and should be tiny. You might prefer to have the cover not returned by regular mail; you might then not address the cover. Most collectors prefer these unaddressed and the chance of covers going through the mail being overcancelled with the usual spray cancel is pretty high. You'll then have to provide a return envelope of a proper size (with postage) for return. It wouldn't hurt to send everything with a short note saying you're providing an envelope for return.