How hard was it to find classes that transfer?

Talk to your academic adviser. They know your classes best. For me it was actually really easy to find one that transferred. There are programs for everyone!

Travis is right, your academic advisor as well as the advisors in your study abroad office will be able to help you, even if you think it's a credit that won't transfer. Before going abroad, the classes you plan on taking will be approved by your college, making it easier to get the credit when you return. I personally had a little trouble getting one of my classes to transfer but in the end my college, the study abroad office, and the department that dealt with that specific class were able to clear me for it. They're super understanding. No one wants to see a credit go to waste when you went through all the trouble of studying abroad just to get it!
You also have the option of taking some of your elective classes abroad if the ones in your major don't seem to act the same on your campus as they would abroad.

The more you meet with your advisor and study abroad office, the less stressful this process will be. I remember emailing my advisor hundreds of times as well as a representative from AIFS (the program I studied abroad through) to make sure I could get credits transferred. Most programs will send you syllabi from past years for different courses which you can then forward to your advisor and the credit transfer office to see what each course can count for. It will be different for each school and each major, but I made it work as an engineering student and I studied abroad in Florence where no technical classes were offered for a whole semester!

Classes transfer easier than you think, even if it is just a "humanities" credit or something of the sorts. Most of my classes transferred as Gen Ed requirements, which is fine because I was able to take classes that I needed to graduate in another country! As everyone else said, your study abroad advisor will become your best friend with class transfers. I recommend getting the go-ahead on transfers earlier rather than later!