http://www.dolphinmusic.co.uk/page/shop/browse/category_id/24b56b6045d06772714ca00fec8642f3 for VSTs. http://www.timespace.com/index1.asp?ID=prosamples for samples. Just select the format you require in the format box and click search. (Most have mp3 demos)
Don’t forget our own Synthzone http://www.synthzone.com/cubase.htm http://www.synthzone.com/softsyn.htm http://www.synthzone.com/ for more resources.
Also check out the manufactures site as most of them have trial versions of their VSTs.
Hope this helps

Bill