ok, you say, large vendors spend time in crawling this sites because of reviews and want to sale. but i think you also want to sale your produkt, so i think you must also watch some public websites, you can create automatic tools and they can download the stuff for you, or perhaps you can also get in contakt with site owners and they send you the stuff.
its ok for me to say, you want not to spend to much time in such lists at this moment, but when you can get this malware so easy you have to take it i think.
i also think vendors sharing samples to, perhaps get in contakt with other vendors perhaps this is usefull.
i also think you need perhaps honeypots and so on, all what helps to detect malware before an user is infected.
and you need good heuristics, this is true :-)