If you would like to find a penpal, there are not too many trustworthy places to find them. However,
Snail Mail Penpals does seem to be the best. With that aside, comes the question, what to send a snail mail penpal?
Here is a list
As for the stationery, instead of the traditional handwritten, type one up at the local library onto unique stationery created by the library computer. While you are at the library, pick up some free bookmarks.
Does your library sell outdated magazines? Donate some money and clip and paste if you would rather create a handwritten letter with some decorations.

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At your local dollar store, pick up some pretty smelling body spray or perfume and spray on the letter and envelope.
Buy stickers whenever you can.
Take photos.
Buy and send postcards.
Buy and send greeting cards.
Create a homemade coupon, such as for "one free hug".
Create a homemade certificate.

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Prayer cards.
Recipes.
With all of this said, you are going to need a system for all of your snail mail. The best way to do this is to use a thrift store, such as the Salvation Army, for all of your filling needs. The following should be useful:
Thrifty Snail Mail Ideas. I hope you make some lasting friends.