When you look at tubes, there will generally be three numbers. You have a 700 diameter tire by 26mm width, so any tube that is set for a 700 diameter tire with a range that includes (or comes close to) 26 will work, i.e. 700x25-28. All manufacturers have slightly different ranges, just pick one and go with it, it is pretty hard to screw up. You can buy them anywhere you buy bike parts, you can buy them at most big box retailers, many hardware/sporting goods shops, or any number of places online.

Also make sure you have the correct stem for your wheels

You're obviously not familiar with British English (or, perhaps more accurately, Australian English), where they say the same thing about us spelling it tire