Some of my best balsa has been from local hobby stores, selected by using a scale.
As I recall, Sig has a grade of balsa that's supposed to be 4 to 6 lb, though I've seen as high as 7 lbs, and I'd expect most to be close to 6. The last time I ordered wood from Sig, several years ago, it was of fairly good quality. They offer c-grain as well.
If you hold a sheet of thin, light wood up to a bright light, you may see pale areas which will be lighter in weight than the dark areas. The other thing to be aware of is that the stiffness is not strictly proportional to weight. Some balsa can be significantly stiffer than average balsa of the same density. You can just flex pieces you've cut out to feel this, or you can get fancy about it. In the latter case, you may be tempted to discard all your wood except for the best piece. Try not to go this crazy. You can find an article on a balsa stiffness test here in Hip Pocket. Also, here:
http://www.gryffinaero.com/models/ffpages/tips/euler/euler.htmlIf you're patient, you can find out more about this and many other relevant subjects in INAV:
https://indoornewsandviews.com/downloads/If my memory isn't failing, there used to be a web page with a whole bunch of articles from INAV, but I can't find it. Nor can I find an index. That's kind of sad, since there's so much stuff in the back issues.
