depending on the size of your frogs, you might be able to go to your local reptile store and pick up some pinhead crickets untill you get some more fruit fly culters, but I don't think that would work for thumnails. I think it will only work for larger species such as tintcorius, terribilis, ect.
2) You do not feed from a batch of ffs until after you have made new cultures from that batch!
3) You keep your "feeding ffs" away from your "developing ffs". This lowers the mite risk (since they normally appear in older cultures - you don't want older cultures near your new/developing cultures).
Also, now is a good time to get cultures of other bugs going.
I have a few dwarf white woodlice cultures and some springtail cultures. When there is a day or two where no fruit fly cultures are producing, my frogs get woodlice and springtails.
Bean beetles are a good one to have as well.
Like everyone else has said; Make cultures every week, and make a few more than you will need. I had a mite issue this summer and had to get a culture from someone in town and order flies when it was 90 degrees all week. Needless to say, after spending almost twice as much as the flies in shipping, I am very diligent about making flies every week.