How much are you willing to spend on this? You can colo at the NYSE or Nasdaq. http://www.nyse.com/pdfs/SFTI_fees_at.pdf
If I had an ATS that would run Linux-based, I might look at using Amazon's Elastic Computing Cloud (EC2) as a budget option. It's more for those who have some technical knowledge. It's robust in that it's the same infrastructure that handles Amazon's business. http://aws.amazon.com/ec2
ec2 has tons of issues, from data mapping with S3 to latency. It's not ready for production applications.