You can purchase grooming bands or little girl's hair bands from the drugstore. You don't want to pull them out....you want to cut them to prevent breakage. They are inexpensive and contain a lot of bands in a band. You don't want to put topknots in too tightly...
My present bag has 750 bands and they cost $2.50 from Walmart.
Here's a link....step 1 does not show up but the rest do and give you a nice visual.
Top Knots
If you want a nice topknot...do not trim the hair at the base of their nose nor give them bangs.