I believe what you are referring to is a "seasonal" insect larvae (worm) ... usually found in the July to August period. I don't know of any company that sells them.
My Grandparents and I used to cut, what my GrandPa called "horseweed", to get worms. Smaller, immature worms were a coppery brown/white striped grub ... while a fully mature one was almost solid white. He called them "Horseweed worms". What the "Horseweeds" actually were, was "Giant Ragweed" plants. They used to grow 6-8ft tall, along the fence lines of backroad farms, that just happened to be on the way to the lake we fished. Here's a link to the "horseweed worm" that I know ... which is actually a "Common Stalk Borer" (first picture). The second picture (Dectes Stem Borer) looks something like the adult "horseweed worm" (common stalk borer) that I remember ... except they had the larger, copper colored head like the larvae in the first picture.
http://www.ipm.uiuc.edu/bulletin/art...rticleNumber=8
Another worm (European Corn Borer) is known to bore into Giant Ragweed, when corn isn't present or is present in close proximity. Here's a link with some pics - telling about the European Corn Borer --
http://www.btny.purdue.edu/weedscien...ECBonGRW04.pdf
And here's another link, showing the Southwestern Corn Borer.
http://www.uky.edu/Agriculture/Entom...rops/ef108.htm
Any of these larvae could have been what I grew up calling "horseweed worms" ....... and all I can tell you about them, is --- they will flat catch fish !! Channel Catfish, Bass, Bluegill, & Crappie would all eagerly take these worms. Good thing too ... since you stood a real good chance of getting quite a few "chiggers" on you, while cutting down the Ragweed stalks
to find them !! ...........luck2ya ..........cp