Datacores and Decryptors drop from exploration Data Site like Central Blood Raider Sparking Transmitter on the one hand, and in more massive numbers from Faction Warfare Loyality Point stores. R&D Agents also provide Datacores but at a much slower rate. Depending on how much time you invest in exploration and running the data sites, you can satisfy a big portion of your consumption via those. Buying them on the market is usually quicker and more convenient.
Remember that you now can use a multi-run BPC for intention and each invention attempt only consumes 1 run off that BPC. You lose 1 run of such a BPC and all the materials that went into the invention, ie. datacores and decryptors (and in case of T3 invention, some salvage materials).
Yes, they are. You can only sell unmodified Blueprint Originals on the market. Modified (researched) Blueprint Originals and Blueprint Copies can only be sold via contracts.