I've never seen an injection available at Academy. If you are set on injecting, I'd go with a no sodium beef broth and add some of the rub you intend to use as long as it doesn't have too much sodium in it. Simmer that for a while so that the flavors blend. If your rub has a lot of stuff that could clog a needle I'd strain using cheesecloth.
Let us know how everything works out, no matter what you do.
Edit: Be careful pumping that thing up. Cooking hot and fast, you can blow it up(almost literally) if you inject too much into it. Ask Phil about my brisket at a contest where we later found out that we were cooking a little hotter than we thought
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Shut up, and cook!!!!