Have I been summoned? Lol jk Whatever your interests are, look there. If you're religious, go to church, if you're a geek go to a Comicon, if you're a nerd go to a library/school, if you're an athlete go to the gym, etc. Sure opposites can attract, but it's easier if the person shares common interests, rather than just "tolerates" your passions and hobbies.
Church - choose her carefully. get into social groups, lead events and get exposed to move females. Through friends and social functions - choose carefully eharmony - I had one or two good matches there. other web sites... I know someone married off match. I've had no success on dating sites of late so I don't recommend them. GAG - there's some quality women here, sift through the ones you don't want for the diamond...
that depends on your definition of 'marriage material'. if you're looking for someone to cook, clean, and raise kids, you'll want to look in different places than if, say, you want someone who has egalitarian notions of marriage and/or doesn't want children.
Try social things like cooking classes or ceramics or some other type of class that interests you. You should try things that you are interested in so you'll find someone with something in common. Stay away from bars.
Definitely not a bar that's for sure. Don't try tinder either. You can try online dating but the risks are pretty high there too. Ummmm... imagination? I guess you just have to go through a lot of people to find the one because it takes time.