I never bought a specific product, although I think you would find one on a baby wearing site, but they might be quite pricy.
What age are we talking about and what sort of baby carrier?
When ds was small and on my front, I bought a cheap poncho from an outdoors shop that is meant for hiking, so it can cover a big backpack. This could be fastened up to the back of ds's neck and then if it was really raining I carried an umbrells.
When he moved on to my back, I just dressed him in an all in one waterproof set (muddy puddles brand) and wellies, and carried an umbrella too.
- A Waterproof cover that covers the baby. You were your normal waterproof coat unzipped or partially unzipped. Something like this http://www.naturalconnection.co.uk/hoppediz-3-in-1-fleece-cover-p-236.html
- A full on babywearing jacket. Brands are Kindercoat, Mamajacket, Didymos etc. They are pricey though (although resale values are pretty good if you sell them on babywearing forums in Autumn)
- DIY it.... a too-big poncho or rain jacket. I use my husband's rain jacket....it is big enough in the shoulders and body to fasten around the baby on the front, or to poke both of our heads through at the top in a high back carry and fasten at the front. Doesn't look very stylish but does the job.