There are several open source shims on thingverse that may be printed, they are also available to purchase online, periodically. The two major "upgrades" for the xiaomi m365 (what bird uses, is about $500 shipped online) is the shim for the folding mechanism, and then a sort of wear plate for the inside of the rear fender to protect the brake light cable. Both bought together usually cost $10 usd.
Typically the shims come in packs of three, depending on how much gap is present in your folding mechanism.