Tojo's right about DIY solutions; I think we have one or two threads here about this very topic, how not to tip over bottles.
I use the caps from used-up rattle cans for some paint bottles, like the old Tester square glass bottles. The inner ring that protects the nozzle fits those bottles perfectly.
For bottles and jars with a larger footprint, I use the caps from laundry detergent jugs.