Yeah, the core set does come with too few channeling markers. I bought an extra core set however, not just for more channel markers of course, but for more core cards. But it did have the added benefit of offering more channel tokens, and action markers I repainted myself to now have 4 different sets.
Not that I suggest another core set to ease the channel markers problem. But if you're crafty at all, you could print and glue some of your own using some of the damage markers. Or can buy a package of small craft cubes and spraypaint them pink and have a small container or bowl of them to use as mana. 1 cube=1 mana.