What a frustrating situation.
It sounds to me like you really do need to consult a local lawyer on this one and actually start taking steps to increase the pressure. If they really did use your art, and you didn't give them permission, then local counsel will be able to apply substantial pressure on them, because that could very easily be a strong copyright case. You might similarly have a contract case, if they made oral or written promises to you about the work.
It sounds like you've done the right things in trying to resolve this without a lawyer's help; but, if that process has failed, you really do need to get real representation and start to push them hard. A letter from your attorney, laying out the law and credibly threatening litigation, might cause them to wake up and treat you more fairly. If not, as you suggest, you might need to sue, unfortunately.
p.s. There are other things you could do -- like go to the Better Business Bureau -- but I doubt those would be effective, and they might make you look weak (ie unwilling to hire a real attorney so trying second-best things.)
Sorry not to have better news. Yours is one of the rare questions I've seen where law might be the right answer.
Good luck.
Honest? I'd be surprised if Hot Topic gave you anything, as they have no connection to you and no reason to believe you. Most likely, they would either toss your request or pass it back to the manufacturer, who then might just get annoyed at you for going behind their back.
I don't know if there are any legal problems with asking for the data, so long as you do not mislead them when you do so (ie be honest), but I doubt that's worth your time, given what I said above.
Sorry again that you're stuck in this bad spot. A local attorney should be able to help you drive this to resolution, and hopefully not at too high a cost.
Law Professor
Professor of Law at Top-Tier Law School, specializing in patent & copyright