were could I get a new logic board for my iPhone 8 plus

While most logic boards are repairable, the iPhone 8 plus board is simpler to work on that the next generation, the X and anything after that, as they are dual layer boards and are not simple to fix.

That being said, if you're sure your logic board is the problem and that it can't be repaired, there's literally only one place that makes iPhone logic boards, and that's Apple. There are, however, a lot of places you can buy logic boards that have been salvaged from iPhones for one reason or another. For yours, if you want the Touch ID to work, you'll need to buy a logic board that comes with the paired home button. There seem to be a lot of them available on AliExpress and I assume eBay as well. The main thing to be careful of is to be sure the seller specifically states that the board is not iCloud locked, and not SIM locked unless the carrier it's locked to is the one you plan to use. Here's the first example that came up on a quick AliExpress search; the price varies depending on whether you get the Touch ID or not, and how much memory it comes with. A 256GB board with Touch ID will cost you around $110 USD.

100% Original Unlock 64gb 256gb For Iphone 8/8 Plus Motherboard Free Icloud With/no Touch Id Logic Board Full Chips Test Good W - Mobile Phone Antenna - AliExpress