MAKE ENERGY CARDS.
Cards that have activation codes on them, when activated you will get CE. Available in Best Buy, Target, 7-Eleven, Shoppers, ETC.
I'm in my early teens. Not old enough for a job, no credit card, can't write checks, not good with this whole mailing thing, and run on a monthly allowance. I'd be more than happy to be a paying customer for OOO but begging my parents to buy me CE isn't going to work out.

Just select the PayByCash and you'll see they are accepted.

You can buy small amount of energy with your mobile phone.

UGC and Pre-paid credit cards are found at 7-11

Like Dogrock said, that's the way to go for teens with no credit cards. I used it myself. You can buy $10 for 3500 CE or $20 for 7500 CE.
Ahh I see. Thank you all for the help!
I went on the Ultimate game card website but there is no spiral knights there.

I bought a starter pack with a ultiment game card. I'm dissapointed that the battle pack is 25 dollars instead of 20, all the game cards I see are 20 dollars, so I couldn't get "just" the battle pack.

redroom100 - The site doesn't update often to add what games they are supporting, as far as I can tell. Don't bother with looking up whether UGC is supported on the UGC site, always look whether it's supported on the game site that you want to put money on itself.
I don't think the game is big enough to attract stores like that. They're crossed 1 million accounts, but I think games like Runescape and Gaia have at least 5 million (I'm pretty sure those numbers are too low). But yeah until we hit something like that, I don't think they can sell to stores like that.