Hi,
Right ! You will find ready made ( immediately available ) cultures at Repxotica on LI.
You might call before you head there.

You will find all the supplies at Viv in the Mist.

IMO , if you want to get a good start/ stock on flies , may I suggest you place and order with NEHerp.
Purchase 2 or 3 cultures ( or more ? ) that are 'ready' . As soon as you get them ... make a few new cultures.
Keep in mind they may not ship because of the cold weather

What culture media are you using?
I use Repashy - only - after trying every mix, including several home made recipes!
Keep in mind this is one area of PDF care you need to 'get right' ..... ....and NOT skimp - as their food must be available.
Keep a strict schedule for making new cultures. ie weekly

In the winter / dryer months , the cultures may dry out a little faster. You could try misting ( carefully) with distilled water.
Try adding a little additional water when you make them. Let the newly prepared culture cool completely before adding the flies.
I prepare my cultures one day and add the flies the next ( cover them ).

If you spray w distilled water---after the spray ----look in to see if you still see larva moving around.
You could add ( squirt/ in) additional media if you think that it's low the?
Check to make sure there are NO mites--- the mites will look like moving grains of sand. If so, move the cultures form all good ones immediately.
To examine more closely for mites look around the rim of the lid.

If , after one month , the cultures are still producing, ( common w Heidi) move them far away from your other /newer cultures.
There is nothing wrong w/ using a culture w/ mites to feed your frogs....but you do not want to make a new one with it or keep near the good ones.
Keeping them too long will increase you chances of getting mites in them.

Try to use a culture that has been producing for about 10 days to make the new one.
This will add flies that are 'ready' to breed in the new culture. You will have less production w immature/ newly hatched flies.

A production routine is important. If it gets interrupted. BUY flies ( if you can ) to get yourself up and running again.

Hope this helps