There are a ton of them, from very large to very small. PHI has about 275 helicopters (it changes from day to day) & there are companies with one. Generally I think 1000hrs is about the minimum, the more the better of course. I don't know of anyone hiring specifically copilots - generally you start out in small ships & work up to medium SIC by seniority. A few companies have tried putting new-hires in mediums as copilots, but it didn't work out. Nobody was happy, & it didn't last long anywhere.
I don't know of a comprehensive list of operators - a new one seems to spring up every month or so. Lots of people seem to be winning the lottery, & running a helicopter business until the money runs out. The major companies are PHI, Air Log, & Era, and many smaller companies. Offhand I can think of Tex-Air, Republic, American, Central, Evergreen, & I'm sure there are many more, I just can't remember them all.