What is the best resource for SC practice questions?

If you're simply looking for practice problems, rather than instruction, you should start with real GMAT problems. The Official Guide is the way to go: https://www.amazon.com/Official-Guide-GM ... 294&sr=8-1

That's going to be more than enough problems for most people's needs. Some of my students have run through all the problems in the OG, as well as the related Verbal Review supplement, and found themselves wanting more. However, you are better off "fully digesting" these problems by identifying the different issues at play, and analyzing each answer choice carefully. If you take the time to do this for the OG and any practice exams you take, and you have a solid source for GMAT-related grammar instruction (I humbly suggest Manhattan GMAT's Sentence Correction strategy guide), you shouldn't need any additional problems.

Best of luck!