Purchasing a water heater dip tube is not something you do every day. The good news is that they are not an expensive part. Many are less than ten dollars and most are certainly under twenty dollars.
For related information see the articles 'Gas Water Heater Troubleshooting' and 'Troubleshooting Electric Hot Water Heaters' for a complete listing of water heater topics.
Finding Dip Tubes Locally
Hardware stores and home supply stores will likely have a few generic sizes. Since the dip tube is designed to force the water to the bottom of the tank, the length is important.
If you have your manual, you can get a part number from your hot water heater. Plumbing supply stores will have more choices for dip tubes.
Finding Dip Tubes Online
Hot water heater dip tubes can be purchased online and you will have a better chance of finding exactly the one you have. Again, using the model number and brand from your hot water tank will be helpful in the searches.
Efforts are being made to improve the performance of dip tubes. Putting curves on the end and special configurations that diffuse the water as it comes out the end are some of the innovations. Try to get a new dip tube that is similar to your old one. A curved one may not go into an inlet that had a straight one originally.
The concept is pretty basic here. You want the cold water going to the bottom of the tank. Newer engineered dip tubes may reduce sediment. If it fits you can use it, it won't hurt anything. Flushing your tank once in a while is not that big of a job, so a straight one is fine also.
Next you are going to want to review the articles, 'Removing a Water Heater Dip Tube' and 'Installing a Water Heater Dip Tube' for instructions on what to do next.