It's not that difficult getting Twitter followers. They just gradually accumulate if you keep logging in and tweeting occasionally. But what I've found is that most of these people are either in another niche, or are just sort of mucking around wanting to get popular (like on Facebook). So they'll just follow you without knowing what you're about in the hope that you'll follow them back.
Of course it's better to follow those who are interested in the same things you are. So, how do you find them? Well, search for a keyword you're interested in. You're bound to find some tweeps with large numbers of followers. If you click on their icons and photos you're sure to find a few others you'd like to follow. But that can take a while, since a high proportion of these people will be like those described above.
A better way is to search for a keyword, find a prolific and well-established tweep who shares your interests then look for the lists he or she is on. That's where you'll find a much higher percentage of followable tweeps, because the lists are clearly categorized. Also, the tweeps listed on them tend to be well established, so if you go to their sites you'll see that they're on other lists as well - and quite long ones, too. So you can just hop from list to list, finding a large number of suitable people to follow pretty quickly.
Also, you'll find that a high percentage of them will follow you back. And here's a clue to know in advance if this is likely to be the case: Just look at the proportion of followers to followed. If it's close to 1 to 1, then chances are they'll follow. If it's not then they're very choosy. If you're just starting and haven't racked up that many tweets (and followers) then it's highly unlikely they'll reciprocate.