I am one of those people that mail out a bunch of letters, bills, and pictures to all my friends, families and bill collectors. I don't like to pay a bunch of money for envelopes so I find myself at Walmart. I like the security envelopes because of all the identity thiefs that are out there. I like the added security that they can't hold my letter up to the light and see everything.
These are plain white envelopes with the bright blue security features. It is 4 1/8 x 9 1/2 inches and made in the #10 card stock. The nice part is you don't have to lick the nasty glue to seal it. I have heard gross stories about it anyways. I just pull the strip of paper and close it. Sometimes for extra security I even tape it. I guess I am just too paranoid because of all the things I see in the news.
I found mine at Walmart. I get them 40 in a box and they usually run about $1.50 to $2.00. I am very happy with it and I have been using them for about a year now. I suggest the security kind if you don't already use them.