I am not sure on their nutritional content but I know people have fed these bugs to dragons before. If I recall correctly they aren't that difficult to breed either.
Doing a little research it seems wild pillbugs can contain alot of heavy metals, especially if present in the environment. Too much heavy metals would be bad for dragons. Thus you best bet is to collect wild specimens and breed them yourself. I don't know how long it will take for them to produce offspring or how many (researching more on the species you have locally would reveal this). You can control what they eat and if they are exposed to any heavy metals (for the most part what occurs naturally isn't a problem, it is if the ground has been contaminated by heavy metals (ie pollution, run off from industry etc).
It seems these bugs can be raised fairly easily, as long as they have ready access to vegetation, be it leaf litter, compost or the like. They do need a very humid environment else they die.
-----
PHLdyPayne