Where can I buy a pre-assembled screen/frame

I understand you want the screen that's preassembled with the frame, which is usually considered a better way to replace the screen on Samsung phones. I usually start with looking on Amazon, eBay and AliExpress, depending on my needs with respect to delivery time, cost and quality. Apparently they're colored so technically you should buy the one that matches your phone's current color. In practice any color will work fine, but your frame won't match the back. Here's one from Amazon.

Amazon.com: Samsung Galaxy S10e 5.8” True Original OEM Display Digitizer LCD Screen Replacement Touch Assembly G970 G970A G970F G970P G970R4 T U V W Premium Repair Toolkit + Free Install (Prism Black Frame) : Cell Phones & Accessories

And here's one from eBay.

OEM OLED For Samsung Galaxy S10e G970 LCD Display Touch Screen Digitizer+Frame

And finally one to see from AliExpress.

OEM Full Assembly For Samsung Galaxy S10e G970F U LCD Touch Screen AMOLED Display Digitize Assembly Replacement NO Burn Shadows - AliExpress

Although the iFixit guide will be helpful, the procedure for replacing the screen with and without the frame are substantially different. So here's a guide from the French site SOSav that will step you through the replacement with the frame. If you bring it up with the Chrome browser, the automatic translation will get you through the procedure just fine.

Réparation Ecran Galaxy S10e - Guide gratuit - SOSav.fr

Or there's the German site iDoc with their guide.

Samsung Galaxy S10e screen repair guide

Finally, there's one of my favorites, a Spanish site called Nadie Me Llama Gallina, or Nobody Calls Me Chicken.

Manuales / Samsung Galaxy S10e / Full screen with chassis

That should give you everything you need. Good luck with your repair; let us know how it goes!