I got two breeding pairs of Lucanus capreolus from a nice person on Mantidforums (Agent A), it was trade
I always go to Bugsincyberspace for all my invert needs (if I am buying, usually I do trades though), then Ken the bug guy (I have never bought from them though).
Just watch out on stags, they seem to flip over on their backs and need help getting right side up again all the time. I have to flip my male and female L. capreolus multiple times a day for some reason :\
Paypal is pretty useful for buying online, and fairly easy to use. Or there is also mailing money types (checks, money orders, and the such) but there is the chance of it getting stolen in transit.
For the mold problem, try to give them more air flow in the containers, and maybe they were too wet? Slightly moist, just enough to keep it from being dry, should be enough.
For your D. granti maybe you could post a new thread? They are rhino beetles and not stags, and you might get more answers on a new thread.
Hope some of that helped
And hope you are having a nice day