Firstly - can I suggest to DAR, as he's the mod these days, this might be worth splitting off from the FAQ sticky into a separate thread?
Right - short courses. There are several out there - I have taught on a few myself. The main ones in the commercial market are the UK based UWE CPDA introduction to flight test done at Flightsafety at Farnborough, and the various NTPS short courses. There are a few specialists who will also do a 1:1 specialist course, or organisations doing training in-house.
I've taught on the UWE course plus some besboke training, and sent people to NTPS. CPDA is really aimed at engineers who will engage with flight test, rather than future FT professionals.
NTPS in my opinion is the world leader in these sort of short courses, and whenever I've had anything to do with them they've always impressed me. The calibre of instructor is exceptional, and they're really good at tailoring courses to the students' needs. The military shools are all about meeting government / military needs, and tend to be both more expensive and less flexible - from my professional viewpoint nowadays anyhow.
However - in my opinion, get some engagement with flight testing lined up first, so that you know what you are looking to get trained for. When that is lined up, if resources permit, try and get on one or more NTPS short courses - the training will be superb, but then you'll be consolidating it quickly with actual professional practice. A year later, with no practice, the value of that course becomes far more debateable.
G