I'd like to have two or more bodies for a locomotive so one chassis can be be used for more than one line. I'd especially like unpainted/undecorated bodies. Do they exist at the retail level, and if so, where can they be purchased? I imagine they would have to be specific to a brand, but ya' never know until ya' ask.
The locomotive manufacturer might be a better place to start. I've had better luck getting new locomotive shells (mostly undecorated, but also the occasional decorated shell) from Atlas than just about any other manufacturer. When you're looking, you'll want to look at the Parts Department area of either the company's website. If you're calling or emailing, you'll want to talk to someone in the parts department.
What brand/model locomotive/s are your thinking of? Maybe we can point you in the right direction for a source.
I was looking for some wide ranging answers because I've yet to buy a DCC locomotive, but if you point me towards an EMD SD40 or SD70 undecorated body that could fit most of the manufacturer's chassis I'd probably end up buying one of those to kick off my DCC fleet. If I can afford 'em. Right now it looks like HO scale, but might look deeper into N scale if my current layout testing doesn't trip my trigger.
I'll keep it in mind for when I narrow down my search for the perfect first DCC locomotive. Thanks for the tip.
Don't overthink it. Looking for the "perfect first DCC locomotive" is just begging for analysis paralysis. Pick a brand that you feel has a decent reputation, read some reviews, then select an available DCC loco in either a road name that appeals to you, or an undecorated version if available. For me, in N scale, I started with Bachmann and have been happy with my choices, but I've come to value my Atlas diesels as even better runners.
More price paralysis looking for the best price. I just now finished ordering a Bachman Industries EMD SD40-2 with DCC installed. Bachman has been around longer than me, and despite their reputation as a very low end product, I've read, right here, a couple comments that their quality has improved considerably over the years. Guess I'll contact Bachman and see if they have any undecorated bodies. Good lord, you wouldn't believe what just popped into my mind as I typed "undecorated bodies" - and it wasn't pleasant!