My understanding of the problem is that nuclear sources tend to be nasty stuff and not the sort of thing that one really wants lay people responsible for discarding.
On that note, the idea you're talking about was actually considered for a little while for public use until the dangers were found to outweigh the benefits.
I am wondering if it would be possible to buy or build such a nuclear battery.
Pu-238 isn't naturally occurring, and nobody makes it anymore. The US bought the plutonium that went into the MSL from the Russians, who are starting to run out themselves.
The other problem with RTGs is they get less efficient as they get larger. The one in the MSL only produces 125 watts, which will decay to about 100 watts at the end of its 14 year lifetime. That's not a lot of power.
In almost every country the materials required are tightly regulated and getting permission even in a lab environment is tough work. If you can work out the legalities and safety, I'd be interested too!
http://www.amazon.com/The-Radioactive-Boy-Scout-Backyard/dp/...
I read the book. from what I remember he started with smoke detectors and worked his way up from there. Just a few letters to some supply houses saying he was a researcher or something. he effectively mail ordered his radioactive sources I think.
The lethal dose of radiation is about 3-5 Sv, that is 3 to 5 J/ kg absorbed radiation (times some factors, which describe how the radiation is absorbed and which type of radiation). Therefore assuming full body irradiation and gamma particles, the lethal dose for an adult would be received in a matter of minutes ( 60kg irradiated by 1 W would be 1 Sv/minute). Note however, that a more realistic scenario for a dirty bomb from a RTG involves ingestion of alpha emitting particles for a rather large number of bystanders. ( And therefore the effects vary a lot by the details of the bomb and the radio isotopes used. )
So already with the radioactive material from a rather small RTG you can build a quite potent dirty bomb, rather independent of the exact type of the RTG used.