About the libraries from Spitfire Audio: Spitfire don't really sell their stuff on third party sites, so their own website (spitfireaudio.com) is where you got to go. Unfortunately the 40% on everything sale ended like... a few days ago, so that's a bummer. Unfortunately it could be a while before the next one. One thing you could do is make an account on their website, and wish list the items you're interested in, as sometimes they do a special wish list sale. Meanwhile, if you haven't, you can get BBC Symphony Orchestra Discover for free. It is the free version of BBC Symphony Orchestra Core, the library I used for the brass (and strings etc). You won't get the sound I have, because those articulations don't exist in the free version. But it does still give you an idea of the sound, interface and which instruments are available (a few less in the free version, but most are there).
About the libraries from Zero.G: These can be found on several third party sites, way too many for me to list, don't know much beyond that. So you got to look around, but it's not too uncommon to find a sale on Zero.G's own website.
I hope that helps. I'll try to ping you, if I ever see a sale. :)