Portland Lighting do some very good models, possibly not the cheapest, but good quality – easy to install as well. I have used cheaper ones in the past and had a few problems, e.g. poor threads, or not very good electrical connections. Make sure you choose something that has at least an IP rating above 65.
Try http://www.portlandlighting.co.uk/main.html
As mentioned earlier some of the cheaper ones tarnish within one year, however don’t forget to offer your maintenance service, e.g. offering to change bulbs regularly, keeping signs clean etc. to ensure your customer looks top notch.