Some resources for choosing colors:
https://baboodigital.com/choose-mat-color/
https://www.carteravenueframeshop.com/framing-info/framing-design/how-to-choose-mat-colors/
https://imageframer.net/articles/how-to-choose-mats-for-photos/
And here's the multipack I tend to order when I need new board:
It's pricey (especially if you're just matting one or two things) but for me it's perfect. It's cheaper than choosing colors by hand and I always use up all the board for something, even the goofy color I get here and there.
My other go-to source is Dick Blick, either online or brick-and-mortar. Again, I don't need anything fancy. The regular, cheapy Crecent stuff does fine.
https://www.dickblick.com/products/crescent-decorative-matboard/