If you have the appropriate background, and take the right courses at the right time, typically the companies will find you, but Blackwater (very public loses in the media a while back remember), Triple Canopy, and some of the other biggies all have an open application on their websites, my partner at work got hooked up with the above companies and pulls in 200$ us a day waiting for deployment here in Canada, and 1000$ us per day working.
He is a non-Tier1 x soldier (Canadian Airborne), and of the 55 guys on his selection course, 20 were SEALS, and all failed, including the x-training officer for Seal Team 6. Most guys fail out of selections during the close protection and particularly the pistol drills phase. My pal is exception to the rule, the top 10 companies generally ONLY take x SF/Tier 1 people.