You are looking clothing stores that carry Fred perry clothing in the US?
Urban Outfitters. Nordstrom/Nordstrom Rack. Neiman Marcus. sometimes Saks 5th outlet called Off 5th, usually just men's not women's.
Local retailers that cater to 'hipster' 'mod' 'punk' and other 'subculture' clothing carry the basics. (Polos, Bags, Track Jackets, Sweaters) but for the cute, classier stuff ie. button-up shirts, fancy sweaters, dresses, skirts, trousers, shoes, belt etc your best bet is Nordstrom and Nieman Marcus! (but NM is way over priced might as well buy it via fredperry.com)
Depending on where one lives, there are Fred Perry stores worldwide. If one does not live near a Fred Perry store, he or she could also buy Fred Perry polo shirts at Zappos and ASOS online. Also, high end department stores such as Nordstroms, Saks Fifth Avenue, and Neiman Marcus also carry Fred Perry.
Well, this is a hard one, because a lot of Katy Perry's clothes are custom made (especially the clothes she wears onstage). For her more casual clothes, I would try looking at Ragstock, H&M, American Apparel, Forever 21, or any vintage or thrift shop. You should be able to find lots of garments in these stores under $20.
It is always the easiest and quickest to go straight to the source for a particular brand of clothing, so visiting the Perry Ellis website would definitely ensure the quality of the clothing; however, you would most likely get a much better price for the articles at Ebay where you can find the same brand of clothing, even new, at less than half the price of the original.