I bought it off of eBay. The seller is actually AUTOZONE, but we don't have an AUTOZONE here in town. If you've got a local one they should be able to get you one. The cost was $19.99 plus $3.00 for shipping plus tax.
My original switch had nice detent clicks between the 3 speeds. This new one isn't as well defined. I imagine it'll still work fine but I won't know for sure until I get the truck finished.
My original switch had started behaving the way you describe yours.