Cheap sanded pastel paper?
Perhaps you could make your own version by mixing up a ground that you could paint onto less expensive paper. I am pretty sure Golden makes things that you can mix into their acrylic mediums to add tooth to the surface. If you build up a thick enough base you could hand sand that surface to get the different grades that Uart provides. 

It would be tricky to match the even consistency of Uart but rolling your own seems the most economical approach. It also lets you control the base color of your surface as you can pigment the ground you are applying.