Resources

Here are various options for your letter-writing supplies.

Plain Crane papers, though modern, should have enough weight to offer the right feel.
Zazzle has a selection of parchment-printed stationeries.
Off-white resume paper available at most office supply stores should also do the trick.

Downloadable letter templates are available here. (Small zip file, downloads from Dropbox.)

Resources

Oaths of Britain theEditrix