Where to get purigen and How to regenerate it?
How much is safe/needed for 29gallon tank?
You can get purigen online (amazon.com, evilbay, kensfish) in varying sizes. I would recommend the 1L version http://www.kensfish.com/product2703.html
and then put them into pantyhose pouches. Obviously the pantyhose will need to be changed if you recharge with bleach but pantyhose costs like $.50 each pair haha. You regenerate it with bleach, and then to wash the bleach off you use lots of water and dechlorinator.
You can honestly use as MUCH as you want for a 29G tank. There's really no limit. It's all safe.