Reddit mentions of Repashy Superfly 17.6 oz. (1.1 lb) 500g JAR

I use the Repashy fruit fly medium to subculture fruit flies. (With five tarantulas, it'll be quite a bit of work- they need to be subcultured weekly, but you can use less medium and make smaller cultures if you like.)

Get a clean jar. Prepare and add enough medium that the flies will have enough for a life cycle (2-3-4 weeks). Open your existing culture, drop in some flies (tap-tap-tap, use a funnel, you'll still spill plenty of them), seal it up. A paper towel with a rubber band works, but more elaborate lids are available. The flies will mate, reproduce, and you'll have maggots (ew) in no time. Then flies. Repeat every 3-4 weeks or so.

The first few times you do it, you might want to do several jars, or do one jar one night, another jar a few days later- so long as there are still viable, mature flies, you can subculture them.

You'll have TONS of fruit flies, and the best way to handle them is with small funnels and an olive jar. Put the funnel in the jar, shake up the 'parent' flask (with the flies in it), just left-right-left-right to disorient them. Quickly take off the lid, and tap a few into the funnel. The flies then tumble into the olive jar. Seal up the parent flask.

Rap the olive jar on the counter so the flies drop to the bottom. Quickly invert the funnel. Now you can use it like a salt shaker: hold the funnel on the top of the jar with two fingers; open the little condiment cup with the T in it, and- shake, shake- drop in a couple of fruit flies. Close it up, and move to the next one. You can knock back a hundred in a few minutes.