You can buy off the shelf units, or, like many members on the these Forums, you can make your own, which means you can customize to you exact requirements. When you've got all the bits, it's really just an assembly job. It can go in a case or be flat mounted on a board. You won't get many second hand ones on the market as they are always useful, whatever their size.
You must first decide how many outlets you need, and then choose between 16 or 32 amp or a mixture of both. choose your inlet socket. Mine is 63 amp but you could go for a 32. when you have decided how many outlets you need, you can then buy your Consumer unit. After you have built your case or board, find an electrician and get him (or her) to put it together for you.