I found this on Universal's site:

https://www.universaltire.com/tubes-and-flaps/tubes/b-18-19-bias-tube-nickel-stem.html

The last time I got tubes the only size I could get was a 500/550-19 .  I called Coker and they advised they could not supply a tube to fit the 525/550-18. Universal said they could and then they had Coker direct ship me the 500/550-19. After a couple of calls they both advised the 500/550-19 would work OK. That was a couple years ago and so far I have not had any issues.

@steve-d Thanks so much.  That is exactly what Coker sent me and the cross section sure looked way too skinny!  If they worked for you then I will give them a try.  Maybe the tubes can be inflated so much they increase significantly in cross section.  And thanks for the speedy reply.

@w-david-penniman I would not worry about the cross section but you will need to take extra care when installing the tube so there are no wrinkles or overlaps. I have learned that it's best to know as much as you can when buying parts as not all vendors have knowledgeable sales people. When I first restored my car 25 years ago and bought tires,tubes, and flaps from Coker they sent me offset tubes. Not knowing the difference I gave them to a tire shop for mounting , which they did, but commented they had a hard time to get the tubes to align! Little did I know until a few years later one failed and when I called Coker for a replacement the salesperson educated me, actually by accident. When I asked for the tube by my car model he said they did not have any as ALL they had in the size needed was center stem tubes. Many vendors are so used to dealing with the Ford  people they don't bother to check the application. Once I learned the difference I called them back and told them to send those center stem tubes and I would make them fit!!

I have used 19" tubes in 18" tires several times without any problems. Yes you need center stem tubes as the offset will fail within a few years.

@steve-d Thanks, good advice and nice to have you share your experience!

@w-david-penniman 

To prevent wrinkles in the tube, Dad taught me a method many years ago that seems to work well.

First, dust some tire talc on the tube before installing but... be careful not to use too much as a pool of loose talc can turn into little balls that will cut a tiny hole in the tube if the tube works even the smallest amount inside the tire.  (Had it happen on 10.00 x 20 truck tires)

Install the tube (and flap/liner) and if necessary rotate the tire and tube back and forth on the rim to be sure the stem is straight in it's hole in the rim.

Inflate the tire to seat the beads (with a safety cage or chain well wrapped around the assembly if a multi-piece rim).

Then, pull the core out of the stem and allow the tire to deflate.

Not completely deflated, but nearly.

Add air, install the core, and inflate to pressure.

I don't think I've ever taken a tube out after doing this that showed signs of a wrinkle.

Hope that helps!  🙂

I have used a small dusting of baby powder in the past. If you put a little powder on the tube and then wipe it around to give a very thin coat it helps to position the tube and center the stem when deflated prior to final inflation.

Thanks to all who waded in on this request!  The rims are at a shop now being media blasted, then painted (no powder coating on this budget) and then new tires and tubes will be mounted.  Using the 19" center stem tubes that I was shipped - after listening to all of you it seems that these will work ok.  Thanks again folks for all you input.  All the knowledge out there is impressive...

If you live near So Cal, you might try Lucas tire in Long Beach. 800-952-4333, lucastiresales@gmail.com , 2850 Temple Ave. Long Beach, California 90806-2213