Hot tub parts can be very expensive depending on the type of part that you need, as well as the place that you get it from. Some businesses will probably recommend you to buy the parts through the company who issued you the hot tub. Not all of those businesses will sell parts for them. If they do not, they may have suggestions of where you can purchase them from.
You can also do your own research to decide on the best place for you to get your hot tub parts from. You have to try more than just a couple of sources. You should look at things like price, and even warranty. You do not want to pay a high price on a part when you can get it from some where else for a way cheaper price. You are going to have to make that decision on your own when you do your research.
The best thing that you can do is find a professional to install your parts for you, once you buy them. This is especially true, if you do not know what you are doing. Hot tub parts are not hard to find, you just have to look around.