-- there are many very reputable places on the 'net to buy replacement straps, if you want some suggestions
I (or others) can email them. If you spend perhaps US$90 and up you will be able to buy a very high quality strap.
As for color suggestions, the model of your 1815 would be helpful (metal/dial/hands combination). Mine
is WG with blue dial, and I think it looks great on a dark blue strap, but yours may very well differ: