A number of comments have been posted asking where you can order piezoelectric sensors suitable for use as replacement drum sensors.
For anyone in North America, contactmicrophones.com sells brass piezo discs for about $1 apiece. They also sell pre-wired piezo discs (with the wires already soldered to the discs) for $3 apiece. They ship quickly and cheaply, and if you buy ten or more, the prices drop relatively sharply.
I ordered 10 pre-wired piezo discs for $20 USD, and they arrived at my house about a week later by regular mail. They will be used in an upcoming project, in which a disassembled GH:WT drum controller is used to play with a practice drum kit.
If anyone has a good source for other locations, or a better or cheaper source for North America, please let us know in the comments!