Hi - we found our donor through word of mouth. first we considered men we already knew, and when that didn't work out (the first guy we asked said no after thinking it through for a long time, the second friend we eventually decided wasn't the right fit because he was a super close friend and was involved in our lives in so many other ways) we ended up with our current donor just by telling everyone we knew we wanted to TTC and did they have anyone they knew who might be an option? at one point we even sent an email to a few well connected folks and asked them to forward it to guys who met the criteria we were looking for. we did the whole thing in a pretty lighthearted manner and no one ever seemed put out,in fact everyone was thrilled to help out. the downside is that literally everyone we knew (or who knew of us) knew we were TTC! but we aren't very private so it was ok.
our donor was a friend of a distant friend. in the year since we met him we've become fairly close to him and the intention is for him to have a uncle or family friend role. there have been a ton of bumps in the 1.5 year process of getting to the point of actually starting inseminations, but amazingly enough, my DP got pregnant on the first home insemination with our donor and she is now 7 weeks along.
Depending on where you live, you can find groups at gay centers etc that seek to bring together queer folks who are interested in coparenting or being donors. I have heard of people using craigslist and other sites but i think you need to be very careful. at one point i thought of putting a "Want ad" up in our food co-op, but luckily we didn't need to.