Hello!
Thanks for your question. Quantitative skills are important to demonstrate, as they're an important component of the MBA curriculum. It's hard to say if taking additional courses is necessary in your case -- I'd really like to know more about how low your grades were in those courses and what your overall GPA was. Your GMAT scores are sufficient to demonstrate competence but I do think taking a supplemental course or two would help your profile. I know it can be an investment, but think about it as not just a strategy for getting in, but also as a way for preparing to do well in business school.
As for where to take courses, community colleges are a good source (and perhaps more cost effective). There are also lots of online options, like MBAmath.com and Berkeley extension online. Just make sure the course is one that you get a grade -- being able to show you got an A is far more important than just completing the course!
One more thought -- not sure what industry / role you're currently in, but that's another way to demonstrate quantitative skills. So think about how you can highlight that on your resume and in your recommendations.
Hope this helps and keep us posted if you have any other questions!!