Local janitorial supply store - 55 gal drum of 12% for $150

[waterbear]

It's sodium percarbonate, an oxygen bleach. One company once sold it for non chlorine shock for commercial pools but no longer does because it was not suitable and created more problems then it solved.. The only company that I know of that is selling sodium percarbonate now for pool use is Proteam which sells it as System Support, a speciality product for quickly clearing very badly fouled pools. (It work, btw, but it's expensive and requires a lot of dry acid be added at the same time to keep the pH from skyrocketing.)
It is NOT chlorine and is not suitable for santizing a pool.

Sodium Percarbonate is most well known as the laundry additive, Oxyclean.

It can be a useful product for converting a biguanide pool (Baqua, SoftSwim, etc.) to chlorine in the initial stages of the conversion.

[fishandfly]

Thanks guys, I'll keep searching for carboys.