ANY decent mid grade cabinet company will have linen towers that are 18" deep, as a standard choice. If they don't have that choice, then it's a low grade stock company.

15" might also be an option and is closer to the "sweet spot" for pantry storage - 12"D shelves. Deep enough to store just about anything you can think of from cans 3 or 4 deep to cereal boxes to small appliances but not so deep that things gets lost like in deeper shelves.

My old kitchen had 18"D shelves in our pantry and things were always getting lost. I was constantly moving things around trying to find items in the back.

With a 15-inch deep cabinet, you will have shelves 13.5" to 14" deep, depending on the thickness of the back wall and face frame.

3/4" thick walls: 15"D - 0.75" back wall - 0.75" face frame = 13.5"D shelves
1/2" thick walls: 15"D - 0.5" back wall - 0.5" face frame = 14"D shelves

If your cabinets are frameless (e.g., IKEA), then you won't have a face frame and you gain the space the face frame is no longer taking up.

Do you have an IKEA nearby?

https://www.ikea.com/us/en/cat/pantry-cabinets-23609/?filters=f-measurement-buckets%3ADEPTH_14_15

Many companies have to modify the depth to 18" . So you need to speak to a designer in the store you are interested in so they can price it out.

Sounds like IKEA 15" depth would be perfect...