I believe you can order one from Hermes. You mentioned that your Kelly is black, and I'm assuming it's box calf? If that's the case, it shouldn't be a problem. Just be prepared for a fairly significant difference in the brightness of the hardware.
You can order a kelly strap (thinner strap - last pic) i the leather that matches the bag - I think it is over 700 usd, but I am not sure.
You can check on line for kelly straps - sometimes luxury-shops carry them (I am not affiliated with L-S)
you can purchase a wider canvas strap (if the store has some in stock) - its used for an evelyne, but if you have a kelly, you can ask Hermes to change the hardware to kelly hardware - I have done this on two evelyne straps (BBK is shown here with an all leather togo potiron strap). This is because the kelly (lobster claw) hardware is less thick and will not rub as much on the single ring bag. (Although there will always be some rubbing- found this out from Pierre at Madison) Evelyne straps last year I believe were between 425- 500 usd; 100 usd to change the hardware. If you do not buy the straps from teh boutique, they will be unwilling to change the color of the hardware. Even when you do buy the straps from the boutique, ask before you buy if they will do so. . .Changing hardware is 100 usd I think.
Here are some pics of lobster claw kelly hardware and evelyne hardware. The only pic I have of the kelly strap that came with my BBK was as a bracelet lol. HTH