The only way I know to access custom_json transactions is directly through blocks — I’ve written a library called steem-state (and helper library steem-transact) that uses dsteem to make it easier to make custom_json DApps. There’s 3 tutorials, the first of them being in the README at https://github.com/nicholas-2/steem-state

By the end of the 3 tutorials you’ll have a fully functioning custom_json token!