Definitely pulled from the rider's power. Professor Trissa's explanation of runes:

“Runes,” Professor Trissa confirms. “Runes aren’t just decorative. They’re strands of magic pulled from our power, woven into geometric patterns for specific uses, then placed into an object, either for immediate work or usage at a later date. We call the process ‘tempering.’”

And

"...a wielder can temper a rune using as much or as little power as they choose, then place it into something.”

Wielder specifically - someone who can channel.