Many will work with you to "verify" you without a referral. Verification essentially means that you call from work and give your complete true first and last name, and the provider then calls the main number at work, talks to the receptionist (discretely) and asks for you.

Some providers here also work with DateCheck. Datecheck is a service that essentially verifies your identity; but also provides an avenue where -- once you have seen a provider -- she can sort of vouch that you are on your best behavior.

The best bet is to look around and see who requires what.