Bob built his own easel out of a step ladder because he wanted it to be firm and it could fold up easily.
Eventually he designed and manufactured one for sale, and it is very flexible. You can also take the top off and use it as a table easel.
If that is a little rich for your budget, the Testrite 500 Easel was the one most of the instructors used before the Bob Ross Easel was available. It is very easy to transport and also works well in your home studio.