[paul burns]

I have a Hi Standard double nine longhorn 22 cal. Serial #1124322
with w-102 under the serial #. I'm wondering if there is a 22 magunm
cylinder available for it

 

[Fatstrat]

Only if it has a steel frame. The alloy frame models were .22LR only.
And I wouldn't try it unless you're certain it was a convertible model. The .22 mag bullet is slightly larger diameter than the .22LR. And convertible revolvers are barreled w/bores for the .22Mag. So if your revolver was bored for .22LR, the bore would be undersize for .22Mag. Not a good idea.

 

[paul burns]

It is a steel frame and it says 22cal. not 22LR. Do you know where a
magnum cylinder can be purchased?

 

[Fatstrat]

For parts, Numrich Arms bought most of High Standards parts:

Note that there were a number of Double Nine models, but they all used more or less the same basic parts.

On cylinders, you're probably out of luck.
Gun Parts only has .22LR cylinders left.
In any case, you CANNOT just buy a new cylinder and drop it in a revolver.
Cylinders are individually hand fitted at the factory, and even if you could find a cylinder, it'd have to be fitted by a gunsmith who REALLY knows his stuff.
There's a lot more to cylinder fitting then just getting it to fit into the frame.
Among other adjustments are head space, end shake, barrel/cylinder gap, alignment, and timing.

Due to the expense of finding a usable cylinder and finding a gunsmith who could fit it, you could almost buy a new gun like a convertible Ruger.