Another good place to buy commons is CardBarrel. Similar prices, if not better, than SportLots. It's advantage over the competitors (SportLots, in particular) is that it offers a single-rate price for their cards so you don't have to spent a ton of time picking through each card and making sure you pick the right seller than gives you greatest discount. Just plug and play. And their prices are always very reasonable. I ordered from them once about 4-5 years ago and was very satisfied.
It's weakness is that their selection isn't as good. They definitely have some inserts, especially for newer years, but they are mostly known for their commons. In fact, when I placed my order years ago, the site name was still "Just Commons."
Everything really depends what you are looking for. If you need about 100 cards from a random 90s base set, Card Barrel would be my first stop. If you need like 10-15 insert cards to knock out a set, I would check both COMC and SportLots first as they are likely to have what I am looking for for similar prices. Just pick the cheaper option. Finally, if you are looking for that one super specific, hard to find card, eBay and Beckett are my first stop, especially eBay. I've been shocked at what has popped up on eBay over the years -- cards I'd never thought I'd own.
#COMMONCARDSMATTER
"The 0-2 pitch... SWING AND A MISS! Struck him out! The Philadelphia Phillies are 2008 World Champions of baseball!"
- RIP Harry Kalas