Does it have to be longer than stock height springs?
you could pick up moog springs brand new from a autozone or o'reilly's for $100 and you can put those bad boys in yourself. Put mine in about 6 or 7 months ago and they are great, and no sag!
OR...
you could go redneck and get a bunch of flat wood cut in disks and slide them in on the top of the spring housing and it will jack the back end up just like a spacer lift!