King of Prussia: the mall (plaza side) now has a place selling only loose tea and tea-related items. Prices for loose tea are in 2oz increments. I tell you this so you don't crap yourself like I did when your order of "a half pounda this, quarter pounda that" comes to $57.
Downtown: if it's still open, The House of Tea. Just south of South street, a selection that is awe-inspiring. I have not been there in 6 years or something.
http://www.houseoftea.com/