On Course Cotton Naturals, well under $100 (more like $50 to $70) Really stretchy and comfortable, wide range of sizes, easy to find. The full seats are great, except they don't have fabric under the seat material, so when it wears out, you got nothin'. They look pretty good--not super thin fabric. Royal Highness makes some good ones, about the same price range.
My favorites are TuffRider - the ribbed, or Aerocool! I've recently become quite enamored of the Kerrits F-Low Rise tights, but they ARE tights and fit as such - but they're INSANELY comfy. I could wear them as pjs, I think!