I'll try to pull together some thoughts for an answer to what really has no definitive solution, I think.

First, where you are may make a difference. Familiarize yourself with the laws in your jurisdiction. In the US, with some exceptions, you can photograph anything you can see from public areas where you are normally allowed to be, however even then there are a lot of exceptions - including trademarks in view for example, some facilities deemed sensitive for national security, some buildings are copyrighted, some types of usage (e.g. advertising vs journalism) may require property releases. Perhaps more to the point, you can often expect some uncomfortable encounters with security even if you are legally allowed to shoot. In other countries this may be more open, or (perhaps more commonly) more restrictive.

The second option is to try to get permission. I've been involved in corporate management of relatively industrialized facilities over many decades, and the fundamental issue you face is not finding them (I think you can if you try), but getting permission -- there's no up side for most operators. You have nothing they want, and lots of downside -- you might get hurt, the photos might be used against them by competitors, in litigation, etc. To get permission you need some hook to interest them. Possibilities might include:

• Go through all your friends and associates, and see if any are employed (preferably in a senior capacity) in some facility, and can get you entre.
• Find some that might be advertising heavily. Offer your services for free in return for being allowed to do some art photos while there (perhaps throw in the offer to let them review and approve any such to remove sensitive material).
• Talk to the city/county business development folks in your area. They are often trying to recruit new industry to come to the area, and may benefit from marketing photos, and in turn they will have contacts and may be able to arrange access to local facilities. Again, you're trading your photos for access, just to a different place.
• You might get with a local paper or regional magazine who might be interested in doing a story of some local facility, and who can get you access in return for photos. This is likely easier if there's some reason for interest, e.g. expansion, corporate takeover, etc. But if you have no prior experience with journalism they probably won't even return your calls or email.

Fundamentally the deck is stacked against you. Imagine if someone came to your door and said "I am intrigued with your decorating skills, may I come in and photograph your home, for use in my portfolio. Oh, why yes, of course I'll show it to lots of people, and tell them where you live". Without something more, I think at best people would say no and shut the door immediately, some would report you as suspicious.