Metronome
Any basic metronome will work. You can get really basic and have just a tick, or you can go really wild and get a full blown drum machine. Anything in that range will do fine, but if you go drum machine, you can always use it to play along with on your videos. Your prices are from $10 to $200. Start small and see how you use it first, then if you want, upgrade to the drum machine.
The online ones are great, if you can have your computer with you when you play. I have a really old one from my years at piano lessons with the arm that waves. You wind it up and slide a weight up and down the arm to mark the tempo. It is SO cool, but I noticed that they are really pricey new - like $100 or so. So I am going to pick up a small electronic one, hopefully tomorrow. I am really picky about the ticking sound. If I find anything good, I will post here again tomorrow evening.