I assume you checked at your home base, if you usually left one there, or at merchants you frequently use, etc?
Do you still have your autosave? If you save your current game under a new name, then reload the autosave, your teleporter stone may be near where your character started.
When you enter the castle your teleporter stones get removed as part of the plot, which you will eventually get back after you go rescue Zandalor and let him teleport you to a new location.
It is possible to replace the teleporter stones using the procedure in the topic below.
Lost Items - guide and directory of inventory files
