Good luck. The lawyer question is tricky due to figuring out what is cost-effective for you. But whoever suggested contacting a lawyer in the same area as the equipment (and where the termination occurred) has it right, especially since Louisiana Civil Code rules can be quite different from what applies around the rest of the US.
Seems to me that you ought to be able to find a Louisiana lawyer who will write a nasty collection letter for you without charging an arm and a leg. In common law jurisdictions, what you'd be wanting to bring would be an action for "replevin" (giving you your stuff back) and damages for loss of use. The "theft" angle is something that has to get pushed through a district attorney, as it's the state that has the right/duty to enforce criminal law. I doubt you're gonna have much luck there, but that's a question for your lawyer on the ground to assess. BTW, there are also small-claims court approaches to this sort of thing, but I can't say whether you'd be stuck with doing this down there.
I'd also explore getting paid for that last gig while you are at it - most states have wage/hour laws providing rules for when you should get paid after provision of services. And heck, maybe you have grounds for alleging wrongful termination, such that your lawyer can also stick that in the letter. Much of this game devolves to building up a credible threat of legal costs that incentivizes the other side to settle on reasonable terms, so again, you'll want to chat with your lawyer about your range of options.
And here's a thought - why not explore invoicing the manager for the cost of the items, and try to get them to buy it? all things considered, do you want to entrust packing and shipping to folks who don't bear you a lot of good will, or instead get some $$$ in your pocket, replace as you can, and move on?
Last thought - don't know if the band is set up as a corporation, a partnership, an LLC, or anything at all, but if you have contact info. for the band members, I strongly suggest your lawyer's letter get sent to all of them as well as the manager. Otherwise, you have no control over the way the manager is spinning the situation to the band.
The more documentation you can provide to the lawyer when you first touch base, the quicker and cheaper all of this should be. Ask yourself what exactly are you trying to prove, what documentation do you have that proves it, and what else could you hope to obtain that helps you make your case.
BTW, this is not legal advice, and we do not have a lawyer-client relationship...but as a brother bassist, I hope you manage to nail the SOB!