Aside from online, the only place I could think of is maybe Sally's or other beauty supply store?
You could also maybe clean out some old crappy nail polish from a bottle and use that? Or if not, you could buy the cheapest polish you can find, empty it completely, clean out the empty bottle with acetone, and then use it for your mixture?
I would just go to a place that sells Wet N Wild. Get their cheap clear polish for 99 cents and dump the contents. Bonus if you go to CVS it's buy one get one free.