Target, but the lighted ones were way more than I wanted to spend so I bought a pack of lights from their miniature section, wove it around the pine branches, and zip tied it to the back.
I found this Santa Letterhead on this site. My 5 year old is starting to know what's up (Him: "Mom, why do you and Santa have the same wrapping paper?" Me: "Maybe he shops at Target too, I think he has good taste!") so I have to be sneaky with the letter. I learned how to write in calligraphy way back in the day (I also took Latin, I was a weird kid) and it definitely comes in handy this time of year. You can also go for bonus points and "antique" your letter. My Santa also color codes the presents (Mommy has a pink sticker on the tag, Johnny has blue stickers, etc) to make passing out presents easier.