Ok Shaun,
Wide Angle - Sigma 10-20 f4-5.6 around £400
Telephoto - Sigma 70-300mm f4-5.6 APO around £180
Macro - Nikon have just released a 40mm macro at around £250 but I don't know a lot about that. I have the 60mm at around £350 but would prefer a longer focal length of around 105mm for a lot more.
Low light prime - Nikon 35mm f2.8 around £190
As to where to get them cheapest you will have to shop around. Amazon do some good deals. You could also try your local camera store. I use a store in Carmarthen when I am in the area and find their prices are quite keen.
http://www.carmarthencameras.co.uk/
Be aware though, there are a few dodgy sites out there and if the price is really low.