If you're trying to convey trust (ie - sign up for this grant info / take our diet advice / look at our EDU offers etc), I have found .org to convert slightly higher than .com. People tend to associate .org with trustworthy organizations and official sources. Most people don't even realize anyone can register a .org. But I love that marketers tend to ignore them because I have gotten some sweet .org's in the past and banked hard.
Besides, unless you're really branding (which is less likely in aff marketing) then you're paying for your traffic and/or doing SEO so you don't expect any type-in traffic anyway.
^ This!
You can definitely find some sweet .orgs pretty easily too.
Quick Tip: Add some nice exact match keyphrases in the niche you're interested in to the Google Adwords Keyword Tool. When you've got a nice buttload of them export them as an excel file and do a find and replace to remove the brackets and spaces.
Then paste the whole thing into a good domain bulk checker like Bulk Check It'll show you at a glance which domains are available along with all the extensions .org .net etc.
There may be a better tool out there for doing this but I've found some absolutely killer domains this way.