You have asked a question that touches on one of my pet peeves.

Bicycle inner tubes aren't as easy to buy as you are being told. If this thread lasts long enough, people will start telling you that an inner tube is like a balloon and you don't need to get the one that exactly fits. A couple of posters have brought up the topic of valve type. Nobody has mentioned valve length yet, but that's an issue too. The more confident the sales person acts, the less likely they fully understand all the issues.

1. First of all, when it comes to bicycle tires 26 inches doesn't mean 26 inches. You have posted the 559 size in your post. That's an important number. You need a 559 size inner tube.
2. How wide is your tire? The tire width will be marked on the side of the tire. It will be listed like 26 X 1.5 for example. When you go to buy tubes, it will come in a box that lists a range 1.75" - 2.2" for example. In a pinch you can cheat a little on the tube width range but I don't think you should have to do so.
3. What kind of valve stem does your existing tube have? Cheap bikes tend to have thick rubber Schrader valve tubes like on cars. Expensive bikes tend to have skinny metal Presta valve stems. Just get the one that matches.
4. Is your rim flat across the top or does it have a wedge shaped top? If it's flat, valve stem length won't be an issue for you. If it's pointy, you'll want a valve stem to match the one you're taking off. Otherwise your pump might not work.

Good luck. My wife and I have taken to riding recumbent trikes. They use size 406 and 559 tires and inner tubes. I've taken to buying my inner tubes on line because none of the local shops, including the shop where we bought our trikes, carries exactly what we need.