I found the best price for peanut oil at Sam's.  They sell it in 35# jugs.
I'd recommend you get two...one might not be quite enough.  Don't know if
you like things spicy, but here is my tried and true recipe for
injection...and it is to die for...

Melt one stick (1/4#) butter (no substitutes), add an equal or greater
amount of your favorite hot sauce (depending on how hot you like it).  I use
Frank's Hot Sauce, then add several splashes of Tabasco Habanero Sauce.  I
add garlic juice and onion juice to taste, and several glugs of liquid
shrimp and crab boil.  If you need more salt, I use plain popcorn salt, as
it is fine and dissolves easily for injection.  I think the key is to taste
as you go.  One hint:  I make it about hot enough to blow your head
off...you will find that once injected into the meat, it will taste no where
near as hot.  I like to inject 2-3 days ahead of anticipated cooking.  I
also salt, pepper and shake Tony Chachere's into the cavity and do likewise
to the outside after applying a LIGHT coating of oil.  Then, do all your
injecting from the top of the bird.  I slide the platter into a plastic
grocery bag, secure, and refrigerate, but don't forget to bring the bird to
room temperature before cooking.  My  $.02  ~W