When selecting a provider to conduct the testing, you'll need to consider both costs and the scope of services. At minimum, testers will collect specimens and send them to a lab. The lab then can report the results either to you or to the provider. On-site testing is also available, but at a much higher fee as you might expect. To help defray costs, it's worth looking into a small business coalition in your region, which sometimes offer packages to make drug testing available to small businesses who otherwise couldn't afford it.
Some providers offer post-testing counseling and rehabilitation.
Know the law. Since collecting samples is considered a form of search and seizure, tread carefully. Make sure that your policy conforms to accepted legal precedents. In your neighborhood. The cheapest option is for your employees to have their specimens collected at the testing site, so it is important that they are located nearby.