It really depends on how much you want to spend...you can get die your own sheets for kinda cheap...but they are plain white and gotto do it yourself.
If you want already made ones, your gonna have to go a bit priceir, so tell me what your looking for and ill try to help
The background paper used by Pros in studios in the UK is called Colorama and comes in a wide variety of colours. It also comes in several widths as well.
This link is for UK sales but it will give you some idea of what is available http://www.richardfrankfurt.co.uk/on-line_sales/colorama.html
You should be able to get this stuff - or something similar - from any decent Photo store, but you will definately get it from a pro suppliers.
You could also look at an Art suppliers. You can get cartridge paper on rolls but not in such big widths.