1) Although it would look nice, most people don't bother with piping on these do-it-yourself trailers. It simply adds another level of complexity.
2) JoAnn Fabrics sells Sunbrella fabric, Crypton upholstery fabric, Waverly fabric...etc...etc. You can look at what they have online, too. (Although some of it is good, Walmart’s fabric is not usually high quality. Also, if you need more later, you may have a problem getting it, if it came from Walmart.)
3) You get what you pay for; but, sometimes you can get it on sale!
4) If you don’t have a lot of fabric shops around you, remember that you can get nearly anything you want via the Internet. However, you may want to see if you can get a sample of the fabric sent to you because you can't really see the thickness or true color of the fabric online.
Once you decide what you want, shop around on the Internet to see if you can get that product for less somewhere else.