Shrewsbury Cakes (RETIRED)

You can substitute about 1/2 tsp vanilla extract plus a tablespoonful or so of water for the rosewater, but the cakes won't taste the same. The rosewater imparts a really delicate floral flavor. If you can find orange water (which is with the spices in my grocery store; it comes in a blue plastic bottle), I think that would be an even better substitute than the vanilla.

Edit: You can find rosewater in an Indian or Middle Eastern grocery (or in that section of your grocery, if you have one); you can also try Amazon.com, I've found it there in the past.