Unless you pony up and subscribe to Rivian’s service portal (for non-factory repair shops) the only way to buy new replacement parts is from your Rivian SC. Used, you can try salvagers. Many of them are on eBay.
I broke my original just like you trying to change the complete wiper blade. Luckily my neighbor got me a good used one from the spare parts bin. After that I only change the rubber wiper itself, so I don't risk damaging the fragile plastic. You can get a replacement rubber wiper for under $10 on Amazon. Just unbolt the assembly so you can change the rubber on the workbench and put it back on without damaging anything.