I have a 1989 Ibanez RG570 that needs to have its truss rod adjusted.
Unfortunately, I do not have the special socket wrench that came with it.

I assume it is metric because the guitar was made in Japan.

I believe the correct size is 7mm, but I want to make sure before I buy one.

Can anyone confirm this?
Just buy an entire Allen wrench set and or socket set, I highly doubt you can find just one anyways, it's also cheaper and more useful to just buy an entire set. Unless it doesn't use Allen wrenches, I know some of those Ibanez's had a specific socket end that you could only get from Ibanez.
I wouldn't say it's the best place... For a part like that you'll be hard-pressed to find someone selling the same thing for less than $5.

All Parts seems to be more expensive than usual, but I've been looking at the same wrench and checked Ibanez Rules first. Rich sells them for $5 as well so I'd say get it through All Parts.