For Heckle and Jeckle, the short answer is -- you can't, at least, I've not found any "official" DVD releases from Terrytoons (Paul Terry Animation) or its current owner. They did have some Terrytoons compilations (which included the mischievous magpies) through Amazon for streaming, but they are not available at this time. (Will they come back? You might check Amazon, from time to time). I love H & J too; I remember having old movies of their cartoons that we watched on a projector. The good news is that there is a selection of Heckle and Jeckle on YouTube; give it a search!
Space Ghost is available through Amazon. If you buy it, review the manufacturing information carefully, as it could be a "Made on Demand" disc, instead of one released by Warner Bros. Home Video (which no longer produces them, I believe.