Hi Bradley,
I have some information for you from my American friends. Firstly all companies with 15 or more employees in the US must provide health insurance for their workers. Unfortunately the bad news is that you have to pay for the cover (by deduction from salary) and that the insurer may choose not to cover any pre-existing conditions. So that is pretty much the same as the UK private health companies. In some cases the deal the employer has with the insurer will mean that the insurer must cover pre-existing conditions, but this will be at a cost - and that cost is always met by the employee not the employer. All the employer has to do is have a scheme in place, not to pay for it as well!
If you do wish to (or need to) arrange your own cover then the biggest insurer is Blue Cross/Blue Shield. Others that have been recommended are United Healthcare, Health Net, Aetna, PCHS, Amerihealth, Oxford, CIGNA, and Qual Care.
Hope this is of some help.