The National Association of Estate Agents can provide details of its members. Letting agents will charge a fee if you rent a property through them, but reputable agents are subject to good practice and quality checks which independent landlords may not be.
National Association of Estate Agents tel: 01926 496800
Here are some useful websites with property advertisements:
Shops such as newsagents or supermarkets have notice-boards advertising accommodation, some of which may be at the more affordable end of the market.
Publications such as Loot have accommodation advertised daily. There are many local papers throughout the London area which advertise accommodation in their areas. Try visiting an area you are interested in and look in local newsagents and libraries for copies of local papers.