I need to find a supplier of a semi recessed vitreous china corner shower shelf the type that fits into a cut out in the actual tile.

Have you visited any of the large tile store lately, I have seen ceramic shelfs in all of them. Sounds like you might be trying to add a shelf to an existing wall of tile, right?

Home Depot and Lowe's carries them. They are made by Dahlia Tile, I believe. Cost: around $12.00.

Do you have a cut out in your tile / shower already ?

KBRS online, you can get Acrylic, Corian, or ready to tile niches & shelves, custom or standard sizes. They work great.