If you are on a budget look for stuff on Craiglist. I got my rear folding baskets for $5. You can also do a number of DIY type things. Do you have a rear rack? If you do then you can find all kids of ways to attach things too it. Crates are pretty popular and sometimes there are wire baskets you can find cheaply that you and attach as well. you can get alot of your things to you cheaply if you take a bit of time and get creative.

What do you usually carry or what do you want to carry?