Getting old Humble Bundles

Included both steam and origin keys, for games on both platforms.

ask around on a trading site, no guarantees

steamtrades.com

Steamtrades or some Russian reseller sites, don't remember names now.

almost all games were for Origin, some of them gave you a steam key as well. there were some exceptions though... (BF3 or The Sims for example)

I have all my Origin keys from the bundle UFT if you're interested.

Hunble? Are you serious?

Those bundles were only for a limited timed charity event. The only people who profit from buying a past bundle are resellers, not the developers, Humble, or charity.

You can find such people on steamtrades (only deal with people with good rep)
Humble Origin bundle had steam keys too

If you missed a bundle, there's no way to buy copies directly from that bundle site. However, many people buy multiple copies of bundles, and save them for later. After some time passes, they can sell the games for a profit to someone who missed the bundle - it's still cheaper than the steam store prices, after all.

We don't do any trading here at steamgifts, but you can try steamtrades.com or several other places. Just be careful if you are new at trading, because there are many, many ways you can be ripped off. Remember that the internet is full of liars, just like real life...