A small clipboard is super affordable, but you can also find them in thrift stores or when family and friends have desk/office clean-outs. You might even find 5+ in the office clean-out scenario (cough, cough). I do love the plain, mute background of the standard masonite board because it makes colorful artwork pop.
My children like to display their work on clipboards. In doing so, they become portable frames for showcasing the work they are most proud of.
I love to cozy up with a board on the couch, and then I can bring my drawing to my studio table for some finishing touches. The visibility of the boards help me to keep my works in progress truly works in progress.
You'll most likely find clipboards if you wander through your local office supply shop. Or, you can find them here (this is an Amazon Affiliate link).
I liked reading Austin Kleon's take on clipboards.
These DIY rustic clipboards on A Beautiful Mess.