I would start with rightmove.co.uk, they're a nationwide property website for both rentals (or lettings as they're known here) and sales. Doing a search for the areas you're interested in will show you the number of estate agencies in the area and give you a sense of the prices. All of the agencies for the area you're interested in are clustered on Allerton Road. "Your Move" is a monthly property magazine that's a glorified estate agency listings. Most of the listings you'll find in it are for the city centre, not the areas you want, so it will be of limited use, though flipping through it wouldn't hurt. Don't know if it's available online, but a google search will probably tell you.
After four months of house hunting, I generally despise most estate agencies. The one we're dealing with is all right, but none stand out as being much better than any other.
