Every gym I've ever been in has at least one "trainer" responsible for the floor (making sure the weights and other equipment is tidy is usually what they're walking around doing) and they're usually more than happy to provide a spot if you need one.
I've used them a few times when the gym is really slow and I don't want to interrupt the other lifters "workout". I thank them afterwards and have always gotten the same type of reply: "No problem... that's what I'm here for. Let me know if you need another"