I purchased this from Amazon. Got it in blue to match the P20 Pro. It's pretty nice.
Huawei P20 Pro Case, Arkour Minimalist Ultra Thin Excellent Grip Anti-Drop Full Protection Hard Cover Cases Slim Fit for Huawei P20 Pro (2018) - Smooth Blue
For when I'm working I needed a holster. Previously for my Droid Turbo (may it rest in peace) I had a Blackberry holster that was just super. Leather, really well made with a nice felt inner lining and a magnetic flap with a 360 belt clasp. So I started looking. Amazon to the rescue. I purchased this leather holster that has all those features and it was really inexpensive, like $12. Yes, it says it's for the Blackberry Key 2 but I compared the specs and they are virtually identical. The P20 Pro fits in the holster with the case I mentioned above. I'm currently working on getting a sleeve as I like to just slip the phone in a leather sleeve and slid it in my back pocket when I'm not working.
Cellet Vertical Teramo Premium Leather Case for Blackberry"KEY2" with Magnetic Flap, Fixed Heavy Duty 360° Non-Removable Swivel Clip