Walmart doesn't have a lot of shorts right now, but last summer they had some men's shorts called loose-fit compression shorts that are the most comfortable shorts to wear on a bike. They're so nice and airy that sometimes I have to look down and make sure I really have shorts on. They're a very soft, thin spandex material. Last summer they only came up to a 2x, but they were stretchy and fit me even though I usually buy a 4x in mens shorts.
About wipes, if you think you have more sweat than diaper wipes can handle, you can find wipes like these:
http://www.walmart.com/ip/Assurance-...-48ct/15570955
Neil_B is right about the thrift store...not the most fun place to go but it's great for clothes when you're losing weight. If you know someone who can hem up the pants for you, then all you have to do is find the right fit at the waist.