We use wire pet brushes. They come in a variety of sizes and you can get the ones that are rectangular, just like a carder.
We use them at home and they have them in our school along with proper carders.
Yes, carders are quite expensive. I bought a pair for my granddaughter from a local wool shop. Fortunately she had some slightly smaller ones that didn't cost quite as much and were a good size for a kid. I don't remember the exact amount...perhaps $40?