I've searched the site for info on a locking gas cap, and the consensus seems to be that Stant offers a suitable locking gas cap for [newer models, at least] Tacomas.
I was confused, though, because some people were saying to get the Stant 10509 and others said it was the 11509. I checked the Stant website, and entered the information for my vehicle (2008 Tacoma). [I could have sworn that Stant didn't supply this information just a few weeks ago. I thought I got a "we don't make a cap for your vehicle"-kind-of-message, but that's another story.]
Anyway, an interesting note came up on the Stant site. It said:
"Part numbers found at your local auto parts store may vary slightly due to different packaging. The part numbers displayed on this page are boxed products. An example of the same part in a different package is 10511 (boxed) and 11511 (carded)."
So I guess that means that both the 10509 and 11509 are the same part, except that the 10509 is boxed, and the 11509 is carded. Just FYI.