I used to use DD-X and found a like substitute to be Alta Zonal Pro Gamma Plus. It's liquid, can be used at various dilutions, and one of them (I forget right now - I think 1:12) gives the same processing times as DD-X with, to my eyes at least, very similar results. It did tend to be a bit contrasty with the slower films I used, like PanF+, but as with any new developer we have to kinda dial in the perfect dilution and time for our taste. Long story short: If I was using DD-X regularly and it was no longer available to me I would have no hesitation about going to Gamma Plus instead because in use I have found them to be close cousins.
Joe