Steer Tube extenders are adjustable in the sense that you can clamp the handlebars onto them wherever you wish.

I suppose, one question is what your existing steer tube is made out of? Steel, Aluminum, or Carbon Fiber?

Anyway, a lot of steer tube extenders available. Here is one, but a search should bring up quite a few more.

https://brandscycle.com/product/gian...r-273637-1.htm



Also lots of adjustable stems.
https://www.niagaracycle.com/categor...0-degree-black



Also a number of non-adjustible riser stems. Although, you get some extra adjustment by changing or moving steer tube spacers if you have them.

https://www.greggscycles.com/product...m-258620-1.htm