There are a few ways of going about getting health insurance. Some freelancers that I know get "catastrophe" insurance. It's fairly cheap... and doesn't cover much of anything unless you have a BIG problem. For example if you end up in the hospital with something terribly wrong... after a certain amount of money is spent, your insurance kicks in.
If you join the Graphic Artists guild, you can get an insurance plan at a group rate. http://www.gag.org/benefits/health.php
Check out this site.. it has a section on health insurance and some other articles that you will find informative about the business of freelancing.
Bottom line: If you are freelancing now and don't have insurance get it now BEFORE you need it... and then it might be too late.