Undergraduate looking for letter of recommendation..

I'm currently a sophomore (math/physics major @ UIUC) looking to do (or at least apply for) some internships for next summer, (mostly fermilab and argonne) but for almost all of them you need at least 2 letters of recommendation... I did some research work with a professor my freshman year and I think I'd be able to get a letter from him but as for the second one I'm not really sure who to ask? I'm tempted to ask my calc 3 lecturer since I've gone to a couple office hours and he knows me by name but I still am not sure if he'd be willing to write a letter, or even if it would be worth asking.. Most of my math and physics lectures are >500 kids so I'm not really sure who could help me out; I figured maybe some of you might have some advice... any help at all would be greatly appreciated