It requires alot of experimental effort on your behalf. The obvious choice is online, by search engines. If you can work the search engines you will become king! - as it has been said so many times before, you need content and target your niche market keywords.
Good luck, I hope you have a massive influx of traffic.
Search engines, hosting directories, effective keywords and pay-per-click resources are all good ways to start promoting your services, but ... before you start there, make sure you've identified your target customer base in as much detail and as specifically as possible (hopefully, within a well-written and complete business plan).
Within the general colo/reseller market you've identified, ask yourself: Are you aiming for a regional or niche-specific market? If so, what other resources aim directly at those prospects? (Think trade organizations, trade publications, regional business groups, etc.)
Also, what competitive differentiators can you highlight to help you stand out from so many others offering similar services? Is there any audience you can offer a unique appeal to? (For example, if you come from a financial services background, you could emphasize how well your expertise will serve prospective customers in the financial services area.)