Your local lumber yard is a reasonable priced option. Buy 4 each 1x2 boards, preferably hardwood. Wrap with fabric strips, leaving a flap to pin to. 4 large C clamps will hold your boards together at the corners. This is how my DGM made hers, now mine, about 100 years ago. I have made my own smaller versions to do baby quilts, and plan to make a king size set the same way in the near future.