Any decent "hard" shell would work. The Marmot Precip jacket is a common choice, and can often be found on sale for less than $75. If you want to move up in price, you can pick up a Goretex Pac-Lite shell on sale from someplace like Sierra Trading Post for less than $150. Golite, Marmot, Montbell, Patagonia, etc., all make good shells.

What are you going to be doing that requires a down jacket and a shell? I generally don't use them at the same time, if only because if it's raining it's too warm for my down jacket. But depending on your needs, there are other choices we might be able to offer.