I would suggest sending for the Southwark Council list of registered childminders, it is better than the Lambeth list - more detailed. (But send for the Lambeth one, too). Some MN-ers have used Cherubins Nursery on Palace Rd and liked it.
There is a very good child minder in Mayall Rd but I can't remember her name, you could ask at Effra Nursery, and have you tried posting on the Urban75 Brixton Forum for childminder recommendations?
Or try SimplyChildcare here you subscribe for 3 months and can place your own ad and see all the ads placed. Have a look at the finding a childcarer search, you can see previews of all current ads, but not contact details unless you subscribe.
It's hard finding a place for a baby, nurseries are expensive and childminders can only take one child under the age of one, so places scarce. But you might find that you could afford a nanny with her own child, which is cheaper?
It gets easier and cheaper as they get older. Good luck! But you need to get going, it's hard finding childcare vacancies.