Where can I buy $20 PSN gift card for maximum $22 on PayPal

Amazon?

G2A.

If it's for yourself why not top up your PSN Wallet with paypal directly?

https://www.paypal-gifts.com/uk/

^ should be here but it isn't lol ... every average grocery store sells those offline
(also the mainstream electronics discounters they don't offer them online, at least here)

Gameladen has a few options, you can check if there is one for your country.

Maybe PlayAsia?

If you're on the US, you could use this.

For that price only giftcard re-sellers in reddit or maybe giftcardgranny but I'm not sure they track those.
GCX

Maybe Best buy, but I don't know if they have digital ones.
Just be sure you buy for the right region.

OP, if you are still hunting, I'll recommend Greenman Gaming.
https://www.greenmangaming.com/games/psn-credit-20/

I bought a few there when they had a discount running, but they are selling for gift card price only, no mark up.

You will need to VPN to access the page though, as it matches you with gift cards for your region, and I assume you want to gift this to a US user.