Most LFS just plain don't pay attention to the goldfish. Think about this if these fish are going to die at some point why take care of them? So if you want to have nice and healthy feeders That will almost positively not be diseased i would breed your own feeders. If i were you i would breed convicts. They reproduce very quickly. But if your arowana is huge than you should probably (as well as the p.bass) find a quicker growing fish. But if you don't mind feeding .5 to 1inch fish to them instead of 5 inch fish i would do convicts.
if you buy feeders, have them in a quarantine tank for at least a few days with an ich dip and high heat. a 10 cent goldfish can kill your most prized fish. not worth it.
A decent quarintine time is 4 weeks, anything less and ur still guessing, why not feed pellets and silverslides as an occasional treat, imo u will reach way better growth with a high quality pellet like NLS