Click on the iMG cloud button on the top left of your screen, then choose Upgrades. The Brain Capacity cards may be purchased there. If you don't see any, you will have to reshuffle until you find some.
You can keep learning even after reaching brain capacity. I do a lot of strategizing so that I learn as many of the longer skills that I want will be done before the stretched brain time. Sixty percent longer on a skill like bubble tuning is much shorter than it is on Mining 5 or any of the 'higher' skills. And some of the short skills like bubble tunung, gassification, etc. are optional. You can still make special bubbles and gas without them and save learning the skills for later.
The way the game is set up, we are encouraged to pick the skills we consider important and specialise. Even to unlearn skills if our priorities change. There is a considerable advantage in not "knowing everything" when new skills are introduced. You can be one of the first to get to do the new things, leaving the brain-stuffed behind.
I am still a relatively new glitch and I have learned, tried out, and unlearned the entire blending and drink mixing skill tree. I am on the verge of unlearning all cooking. I mostly eat plain meat and drink plain milk. When I get richer I'll buy fancy foods from others if I like. It's never too late to specialise and start unlearning unneeded brain-stuffing skills if you want to be lean and brain-agile.
Even those who consider themselves at Endgame have the freedom in Glitch to tweak their skill tree.