Places like West Marine and Defender have teak rails. You'll have to measure the bases and hope they're a standard spacing.
If it is an oddball size then any good local yard or millwork outfit should be able to turn you out a handrail from a teak board. It's a pretty basic shape to form.
Charlie P. P31-2 #80 CATNAP
To travel hopefully is a better thing than to arrive - R.L. Stevenson
I suspect that, if you should go to the end of the world, you would find somebody there going farther . . . - H.D. Thoreau