Most of the ones I've seen have been from the more telco/comms focussed manufacturers e.g. Anritsu (now Wiltron, or visa-versa - I forget), Wandel & Goltermann, R&S, etc. I also recall HP made some, and no doubt others as well.
A quick look around doesn't show anything particularly cheap at the moment, though in the recent-ish past I've seen models like the Anritsu ME538L or ME4510B go for <AU$200 fairly regularly.