To bleach your teeth from a kit, you would purchase one from a drugstore or an online retailer, or else go to the dentist and ask for a tray from your dentist.
When you get a tray from a dentist, he will probably take a mold of your mouth and form fit a tray based on that mold. This doesn’t have to take very long and could possibly be done while you are in the office. The procedure is an at-home whitening, but is done with dentist indirect supervision, so you have a professional that you can ask questions to.
Of course, there are a great many teeth whitening kits that you can use to make and maintain the brightness of your teeth. These kits may contain a tray that is a one-size fits all type of tray that fits to the general shape of your mouth. It may not be an exact fit, but it will do the trick. You may find these either at the drugstore or online and will usually do a pretty good job.
I have seen a few that guarantee a custom fitted tray, where you send a mold of your teeth back to an online retailer where they shape a whitening tray to your mouth. While this certainly would be effective, there are less expensive ways to get a kit, with a generic-sized tray that when it is warmed in hot water, it will form fit to your mouth. This is a little less expensive and can still give you great results.
Drugstores often have one or two different kits for purchase. These are usually brand name kits from oral hygiene manufacturers that stand by their product (that’s not to say that some online retailers won’t!). So a drugstore is always a good place to check out, if you only want to try out big name kits.