First things first: Where are you? It'll be helpful to know that so we can point you to a shop in your part of the world.

As for the tires... There are a whole bunch of different 26" tire standards, many of which aren't interchangeable. There are even two different, incompatible, 26" x 1 3/8" standards. One of them has a bead seat diameter of 590 mm and the other is 597 mm. Sheldon Brown has a page specifically about 26" tires on his web site:

https://www.sheldonbrown.com/26.html

If your current tires list a size in ISO/ETRTO notation, it'll help you identify which you have (without the need to measure). Look for a number like 35-590 or 35-597. The part before the hyphen is the width of the tire. The part after the hyphen is the bead seat diameter, which will positively identify which 1 3/8" standard you need.

The most common 26" tires fit mountain bikes made in the past 20+ years, and those have a BSD of 559 mm. They won't even come close to fitting rims meant for either 26" x 1 3/8" standard. You can identify these because their tire widths will generally be expressed as a decimal number instead of a fraction. For example, 26" x 1.75" instead of 26" x 1 3/4".