You can google something like: inurl:write-for-us.html to make it easier.

Or even:
[your niche] inurl:write-for-us.html

Yep. but be sure that you are sending emails to those who are having blogs of your niche. Check their traffic. If more traffic is for that blog then some part will be for you too. make sure that your guest blog is trust worth and having some authority too.

And to get a list for your guest blogging, first comment on your niche blogs and from that build a relationship with them It is a long term process but very fine in giving results. Hope my words help you dude...