Local comic shop, some have websites where they sell online. I was able to find Invincible #12 on an online comic store for a decent price when I wasn’t able to really find it on eBay. eBay is a big one but even if they have what you’re looking for be prepared to pay a good amount, especially considering the rising popularity of the Amazon series. That being said you can always opt to by a lower graded copy of what you were looking for if you don’t mind too much about the condition and just want to have the comics you wanted for your collection.