Some are titles I have read before and enjoyed, some are things I can get free from the library and others are recommended by friends, magazine reviews or I just like the sound of them.
I spent a few months putting a bid of 99p on EVERY graphic novel on eBay. There are about 30,000 of them listed at any one time. I won surprisingly few but got some really weird things no one had heard of. If they were part of a series then I would try and collect the others in that series.
At some stage I will put out a call to the general public for suggestions or maybe ask some famous people.