YOu should be able to get the needed adapter at any decent size RV store or Camping world or Generator store.. I would not be surprised to find 'em at Wal*mart et-al.
PROPER wireing is like to like that is the bright screws get the white wire dark black and green the green, DO NOT join anything together except at the 50 anp end you can "Bridge" the two legs
NOTE: If you go to the RV/Generator/Wal*mart you need TWO adapters
One is the RV/Generator 30/30 (TT-30 outlet twist lock plug)
the other is the 50-30 RV adapter (50 outlet 30 plug)
Note 2: IN some cases (if you have, for example, and HW-50c) you will get "open ground" errors when using a PROPERLY wired adapter.. That is the ONLY time you join neutral and ground. DO NOT otherwise