You can also use the tracks with a regular van roof...just need to drill holes and seal. The setup IMO ends up a bit cleaner looking than gutter mount towers. There are at least two or three styles of towers that mount in the tracks, both locking and non-locking.
As mentioned, Yakima and Thule stuff are usually all over Craigslist, at least here in So Cal. The gutter mount style has different clips for different vehicles so you may want to pay attention to that when buying used.
2008 E350 RB passenger 4WD SMB penthouse
2013 KTM 350 EXC
2008 KTM 250 XCF-W
2000 KTM 200 EXC
2003 Honda Element