My LOR's came from professors that I got to know.
For example, I took the honors chemistry class for gen chem II (based on chem I performance), O-chem 1 and o-chem 2. This resulted in much smaller class sizes. That allowed me to get to know the prof through office hours. I also have one from a physics prof who taught both semesters, so by the end of the year, I'd gotten to know him very well (played golf over the summer with him). They are all excited to see you succeed. If you're excited about their material and the key it holds for your future, they'll join in. This was my experience anyway. Hope this helps.