I'm too cheap to buy the pre-made ones, so I was scratching my head over the cheapest, easiest way to make my own. I was thinking wood because... well, because.
Then I stepped back and started over. What do they need to be/do? (1) Durable and waterproof (2) Easy to read and write on (3) easy to poke in the soil.
For some reason I was eyeing 2-liter soft drink bottles (I tend to have way too many of those around for some reason I won't get into...) then I opened my fridge and the light went on... okay, yeah, the fridge light too.
Milk! Those plastic milk jugs are made from not too thin, white, transluscent plastic that's slightly rough. All that's required is washing one out (drink the milk first, don't waste it!), grabbing a scissors and snipping away. A fine-point Sharpie writes on the plastic perfectly.
I'm sure someone else has done this, but I had not found the hack elsewhere. Maybe some here can benefit.
I ran out of milk jug (temporarily) since the first was a mere quart jug, so had to cut up a top to a yoghurt container which worked fine also. It's had some distracting print on one side, but the inside what white. Milk jugs are the best so far.