Do they have pace groups for marathoners who are in the 2:40 range? or is it only 3:00+?

Please define "they."

do marathon races have them?

or do you have to hire someone for lots of $?

do they exist for 2:35-2:40ish types?

At Fukuoka, you could pace yourself with the sweeper van.

Close to home, I've heard of some races (CIM?) providing a 2:4x pacer for women's Olympic Trials hopefuls.

the frankfurt marathon had a pacer/pacers assigned to each elite runner, not just the top ones, a few of the slower women elites were trying for to run 2:35-2:40 so i guess you could run frankfurt find out who the women trying to run those times are and what their bib numbers are and when you see her at the race line up alongside her and since she will have pacers to get her to that time, if you run alongside her then you have 2 pacers her and her designated pacer.