Someone posted a link to a way of making them by origami. I got completely hooked but in the middle of winter and ended up using them to light the fire
It's ten quid for an offcut of wood as far as I can see. You can either try the origami ones of if you just want the cylinder ones (easier to make, but not as strong) then any kitchen roll inner, can, bottle etc will do to form the pot.
I've got a mini experiment going to see if you can do your own root trainers, as I had two left over French and two left over runner beans. I put one of each in toilet roll tubes and then cut corrogated card to the same size and paperclipped it, corrugated side in, to the same diameter, and sowed the other two in there. Not terribly scientific but I thought it would be worth a go to see if the tubey bits on the card guide the roots down as roottrainers do. Not that I'm entirely convinced that the nice vertical roots on roottrainers are essential but there's no harm in trying.
I haven't tried the origami squarish ones, but we've made a LOT of the round ones using toilet roll and kitchen roll tubes as a mould. They're great, we made some bigger pots at the weekend, using a pop bottle with the bottom cut off instead of toilet/kitchen roll tubes.
We both had a very smug moment or two when we were transplanting our seedlings from our little paper pots into bigger ones, and then putting them on old food container things, kept just for that purpose
Someone posted a link to a way of making them by origami. I got completely hooked but in the middle of winter and ended up using them to light the fire