We have a folding wooden easel intended for painting or
drawing. Same sort of arrangement - a tripod with two
legs holding the paper and one in back to hold it up.
It might help, however, to have at least one leg be
adjustable if you don't have flat ground. A camera
tripod is usually suitably adjustable, and may be
found cheap at Goodwill and other thrift stores.
A surveyor's tripod is sturdier, and also adjustable,
but probably more expensive.
If you usually set it up on flat ground, having
adjustable legs isn't as important.