Hitch extension source?

the 2.5" extensions fit into your truck receiver, but use 2" receivers on the trailer end.

I run a curt at either 36" or 48" depending on what I am towing.
With any tong weight I really recommend Weight Distribution hitch.
because my weight distribution receive is longer than my normal drop receivers I can can run my extension at 36" instead of 48. And the weight distribution really makes a difference. I had a set of 600lb bars laying around. I think the camper rides better with my 7x14 V nose behind it with Weight Distribution hitch then the truck does without the camper.

I know you want a solid 2-1/2" 18" extension. I've towed our 7,500 lb carhauler trailer many 10's of thousands of miles on a 2-1/2" X .25" wall 22" extension. Just something to consider.

http://allmetalsinc.com/hs2500.html

I got a 48” extension from Etrailer. Works great with towing my bass boat behind my TC

If this supplier doesn’t have what you’re looking for.....it probably isn’t available.
https://www.etrailer.com

What a problem to build one?
In California I had ACE Hardware Truck Accessories supply that had good selection of receiver tubing. Than you buy solid bar at fabrication shop and weld it.
Lot of such stuff on amazon.

You can find an 18" long 2" extension from online vendors lie Tweet'y, Curt, Reese, etc. The 2 1/2" extensions start at 24" and go up from there, so your choices are restricted unless you make something yourself.