I just bought a 2009 civic Hybrid with a bad batter. It only has 120,000 Miles on I just changed the plugs as soon as I got it. What's the cheapest option on rebuilding the battery myself? Were can I find a place to get good cells from?

 

There isn't one.
There isn't one.

(not a typo - same answer to both your questions)

09-11 HCH hybrid batteries were the worst batteries ever made with a 30% failure rate in the first 3-4 years. Honda avoided yet another hybrid-related lawsuit by extending ALL warranties to 10yr/150,000 miles even if outside CA.

You have two choices:

1) Futz around with the battery repeatedly trying things that fail over and over and over wasting money and lots of time until you eventually do #2
2) Replace battery with a quality recondition or new.

I have owned 5 Honda Hybrids. Of those two were '06 HCH. In an effort to build good packs, I tested and reconditioned 14 packs. I was able to build one good one, one barely acceptable one and one that got by with monthly grid charges. The rest of the components were garbage.

I have the capability to fully test and recondition an entire pack in less than a day with equipment used commercially.

I will not work on these packs. Period.

Honda completely screwed up the '06-08 pack with a horrible design that produced literally hundreds of hot spots. In the '09-11 redesign, their "fix" actually made it worse.