Hi Bayoumom,
I bought the grains and the Oxygen Absorbers from Honeyville Grains. They have outlets in Utah and in Rancho Cucamonga California. I think that Honeyville has some of the best prices for Oxygen Absorbers. Even their website price (much higher than the warehouse price) is pretty good.
I bought the pail liners from Sorbent Systems.
Yes, I had heard that you had to use 4 oxygen absorbers for a 5 gallon pail. I read 3 elsewhere on the web, so did my own experiment. I used 3, and all of my mylar bags had a vacuum after I sealed them. ("Look," I said to my husband, "they all suckied!" Suckied. A very technical term, hehe.)
If I were packing items that did not pack closely (pretzels? cookies?) then there would be more air to be absorbed, so you would have to use more. If I were packing a 6 gallon pail, I would use 4 absorbers.
You can do your own experiment: pack up one pail of grain, and use 3 oxygen absorbers. Wait overnight. If your bag has "suckied" then you have used up all the oxygen in the bag, and you are fine. If it didn't "suckie" then you need to use another absorber. Make sure you squeeze out as much of the air as you can before you seal the bag with the impulse sealer or iron.