I don't remember I find such a Formula, but it's always worht the effort to visit each trader each level (they restock items).
What I always did in my previous play :
searching for weapons with that encantment on it and if that weapon isn't what I wish to use I DISenchant it and use the learned formula (via disenchanting) on my favorite weapon
I find more good/needed enchantments on gear than I find the Formula itself somewhere or by a trader.
After taking over the Battle Tower it's worth the effort to first search for 3 different parts of a Zeppelin to give to Page at the Champion Harbour, so you can fast go to Aleroth where you can trade with at least 2 very good traders !
And make a study of all the Blue Teleporters is also worth the effort if you like to trade and go fast from place to place !
Just my thoughts
