Do they still make Shadow boxes, if so where at?
Depending on your definition of inexpensive — Michael’s always has some and if you wait for a coupon or a sale (not usually a long wait) they can be affordable. Frameplace.com is an online option, although I have not purchased anything from here, they do have some. Also the Ribba frames at IKEA come in several sizes and are quite deep. I use these frames on a regular basis for regular framing, but by putting spacers (cut from foam core, or they make plastic spacers just for this purpose) between the frame and the back you can easily turn it into a shodow box. You’ll probably find these frames to be half the price of a comparable size at another store.