My understanding is that for home search warrants, the police have to claim that they have the word of an informant who they know (but they don't have to put his name in the warrant request or otherwise tell his name to the judge, for his protection). And generally, to get a warrant (without other evidence) they have to claim that the informant has given them good information previously.
I think that in some jurisdictions this is not true for vehicular searches. But it is highly problematic to give warrants for truly anonymous tipsters, since the police can of course give themselves any "tip" they want.