Paul has you covered !
He's right --- the best thing would be springs and isopods ---lots!
Seed both tanks!
Perhaps there is a show you could go to?
It would save those darn 'live' shipping costs.

FF
I use the 'leftovers' as a guide to how many flies.
Put in- at least 20 - or more- a day. If there are a lot left over ..then you can cut back as long as you are sure he is not haveing trouble finding his FFs.
If he has had difficulty 'growing' it may mean he was taken from a 'grow out' enclosure too soon? Feed him well.
Put the FF in the same spot every day. You could bury a tiny pc of banana at the FF site to keep the flies around the area.

I feed all my adults PDFs ( fruit flies) daily - following routine:
-dusted daily
-alternated Repcal Ca, Repashy, and RepCal Vitamins
-the adults get Repashy Vit A once a month

however, lots of people don't feed darts daily- I like to. What difference does it make ? ...as long as there are not too many FF in the enclosure -stressing the frogs. Knowing they are eating well and getting their supplements is all that matters

Now and then, I might skip a feed --- so the frogs can clean-up !

-the ,very,very, newly morphed froglets get springtails and isopods
( kept in little 2.5 gallon tanks/ dampened paper towel, a sprig of real plant and leaf litter)
-new springs are added daily
-the older froglets get fed FF 2X daily - dusted once , spring, and isopods

I feed isopods green zucchini and springs bakers yeast- both are kept - very successfully- in the same container with wet ABG, leaf litter, and big chunks of charcoal. I keep 3 large cultures---One is used only to replenish/ add to the other 2; these '2' are used to feed the babies. Works for me.

Enjoy your build.