Where can I get a replacement display?

Since this is a fairly new phone model with an OLED screen, replacements aren't cheap, but then again, they're a lot less than the cost of a new phone. Here are some examples; if you do an internet search for something like, "Samsung Galaxy S21 FE screen replacement" you can find more.

Samsung Galaxy S21 FE 5G OLED and Touch Screen Replacement – Repairs Universe

Samsung Galaxy S21 FE 5G Screen Replacement Kits & Parts – PhoneRemedies

Amazon.com: Ygpmoiki for Samsung Galaxy S21 FE 5G SM-G990U SM-G990U1 G990U G990U1 6.4" LCD Display Touch Screen Digitizer Black : Cell Phones & Accessories

You can also check sites like eBay and AliExpress who frequently have lower prices, but you sometimes take a chance on quality. It appears the color of your phone is a factor in selecting a replacement screen, so keep that in mind.

Instructions would probably be nice too, I imagine. Here's a guide for replacing the LCD+frame assembly.

Samsung Galaxy S21 FE 5G screen replacement - repair guide

Good luck; that should get you started. Don't forget a sheet of precut adhesive to glue the rear cover back on and help restore most of the waterproofing.

Note that you can buy replacement screens with or without the frame; the only written instructions I was able to locate are for the version with the frame, but you can find YouTube videos of the procedure to use when replacing just the OLED display without the frame.