I tried reusing a 6ml dropper bottle but i hard a hard time getting it to retake fluid. I found a useable thing in the beauty dept at Rite Aid but it's a little too big to carry in my pocket. I saw someone doing a video review or something and they had a thin dropper on their keychain that slid out of a protector. That would be ideal but I don't even know what to search to find something like that. I'll see if i can find that video again, but in the meantime what do you guys use to top off your carts on the go?