If you need to search for someone's drivers license number or other sensitive data like social security numbers, driving history and such, it is important that you understand current privacy laws that protect such data. Although these laws are in place, it is still rather easy to get your hands on this data, but you need to insure that you have a legal reason to do so.
The Driver's Privacy Protection Act (DDPA) was enacted in the 1990's and it protects these types of privacy invasions. There are specific reasons listed where you can legally gather drivers license numbers. Officials with the judicial system or prospective employers have good legal reasons for getting at these numbers. While the act has been amended to include many of the technological changes of the last fifteen years, ways to circumvent these safeguards are readily available.
There are many places online where you can access a lot of this type of data - some of it is legit and other is not. Identity theft is a huge problem now and a person equipped with a few pieces of private information can steal money from unsuspecting online shoppers and browsers. Some online sites state that information thus gathered is not to be used for illegal gain but that doesn't stop a thief hidden behind the anonymity of the Internet.
The DDPA does protect against giving out some types of private information - it does not restrict access to certain types of public records. Some of these public records contain sensitive private information which can be then used to find out other, illegally usable information such as a driver's license number. Some forms of Legal proceedings, real estate transactions and driving records are considered public record and may be available to anyone. More often than not, this form of record has the name, address and other data for finding this otherwise sensitive information.
The better sites are going to cost you money, and have some restrictions about what you can search for, but provide a good service nonetheless. Many states have their own laws for prohibiting the access of private data that might be considered sensitive. Hiring the help of a private eye can also help if you are willing to shell out the cash.