As far as getting new "Nitro Express" Airsoft shells, oyu can also try the official Goblin dealers, don't know much much cheaper it would be to get a different brand not intended for the goblin.
Ive also experimented with filling a paper cup with 6mm airsoft BB's with a wad and using it in the .68 caliber paintball shells. Still works but I don't think you get full range. I may have to test it further.
Also I forgot to post the pic for the ammunition types so here it is:
