I live in Canada and have bought CD-3 from Artcraft in the US, and CD-4 from Photogrpahers Formulary, also in the US. Both purchases were a few years ago.
The bigger challenge is some of the colour addenda other than just the developing agents will not ship.
Hydroxylamine Sulfate is one. Equimiolar substitution with the hydrochloride salt in lieu of sulfate is an option, but I am not sure if it ships either, just that I happen to have some.
Basically from my limited chemistry understanding it is an ammonia and sulfuric acid merge, and in quantity is quite a good deflagration agent.