Hi Robin,
You have several options for your tablet available. As you mentioned, yes, you can indeed buy another used one and disassemble it for the LCD. From my reading of the replacement guide, it's going to be the same amount of work to either replace the LCD on yours or swap your motherboard into another system. If you've replaced the digitizer then you already know exactly how much work it is.
Anyway, for your reference here's the iFixit guide for replacing the display on your tablet.
[guide|40537|RCA RCT6272W23 Display Replacement - iFixit Repair Guide|new_window=true]
As far as finding a replacement LCD, there's one seller on eBay right now selling them for US $25.89, which isn't bad for a new screen.
[link|https://www.ebay.com/itm/303642255112|New LCD Display Screen Replacement for RCA RCT6272W23 7 Inch Tablet F88|new_window=true]
I didn't find a lot of others, but if you remember that RCA doesn't make their own LCDs, you can probably look on the back of the LCD itself and get the manufacturer and model number and search for parts using that number instead.
Good luck; be sure to let us know how it turns out for you!