Filaree Farms sells mesh tubes in continuous lengths, and in several colors.

You cut the section you need and seal it either with an overhand knot, or by squeezing a pig ring around it (which is much neater).

GardenLad, thanks!

I don't know why I've had such a hard time finding them, I swear I've dug through tons of websites and called way too many stores in the area. Now that you've pointed out Filaree Farms, I find that Pinetree Garden Seeds also sells them (as a 2-in-1 Harvesting Bag). I had previously just been searching places that specialize in garlic (although I forgot about Filaree), but they just didn't seem to carry them. Thanks for putting me on the right track!

Dixondale or Ronnigers sell these, I forget which.