I need to order 2 of these channel balances, but don't know the brand or size or where to order them from.

Thanks for posting. This looks like either an S351-28 or S390-28. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the image below. Compare both with your old hardware to see what matches best. In either case for a stamp option go with 2730.

Comparison of the S380 and S385 balance shoes.