I use a variety of small bottles in the shop.
One good source is Harbor Freight - they sell 8oz plastic bottles with spouts and snap caps. You can find similar containers at a shore that sells hairdresser supplies. I also recycle condiment (eg, mustard) containers, as well as bottles that originally held spectacle cleaner or shampoo (especially the hotel-size bottles).
There is a concern that the solvent in some finishes can evaporate through the walls of plastic bottles - whether this will happen depends on the solvent and the specific kind of plastic involved. So I tend to use small glass bottles for mixed finishes where its helpful to not allow the balance between solvent and other components to change over time. For the same reason, my wife prefers to buy cooking spices in glass bottles, and I grab the empties for shop use.