We buy Merck silica TLC glass plates (250um, with UV indicator), 20×20 cm. We get them from EMD through VWR, for about $90 per pack of 25 (after discount). I divide each plate into 3 strips of equal size (about 6.5 x20cm) and I cut the strips into individual TLCs as I momentarily need them, the width being determined by the number of reactions (fractions, standards) that I need to run on the TLC side-by-side.
Some people hate cutting their glass-backed TLCs and buy pre-cut ones (expensive and one-size only) or use aluminum-backed plates (incompatible with several good TLC stains). I found that cutting the glass plates is not hard to do and that crappy, worn or damaged diamond cutter is the most common reason for frustrating results. For example, Chemglass sells acceptable diamond cutters for reasonable price. (One has to clean the grease off the tip with some organic solvent and fix the wiggly headpiece with diamond tip onto the handle – by loosening the screw, rotating the handle and re-fastening it). Silicon-carbide disc “pizza-cutters” can be also pretty good – I have been sceptical about them but now I use a plate-cutting contraption that has Si-C disc in it and it works fine.
One common beginners mistake is to double-cut or push hard on the cutter. If the used diamond cutter is sharp, a hair-thin, nearly invisible cut made with one light slide works lot better than wide a trench dug in the glass through a screeching effort. Also, it is important to use some straight solid ruler that does not bend sideways, for cutting/breaking the plate. (A nice plastic one has a Y-shaped profile and marks with different scales.)
If you find yourself next to Home Depot, you should buy a polished marble tile for your TLCs; it is not expensive (something like $5 a piece). It serves as a perfectly flat surface with a sharp edge. Also, the marble (but not granite!) is quite soft – so your cutter slips off the glass plate, the impact on the tile will not damage the diamond tip. The tile is also a good place where to store couple of pre-cut TLC glass strips (facing down). But the tile has to be a real marble, not a glazed ceramic.
[I stole my marble tile at a construction site in Boston and I carried it with me ever since, through couple of jobs, to San Francisco and then back East again.]