It looks like fans for the Steam Deck are back in stock over at iFixit after quickly selling out almost immediately when they started going on sale. Though, I wouldn't expect this to last much longer either. It is hard to say how long the fans will stay in-stock, seeing as how they sold out almost immediately last time, but I would get it ASAP if you want.
When the Deck was first being introduced, Valve was using a specific fan that was much quieter, but just as efficient. Though as release came closer, Valve switched the fans out (more than likely due to production cost or availability) to a similarly efficient fan, but one that made a high pitch noise when it was running at high RPM. iFixit, being an authorized Valve part reseller, got their hands on this original fan and sells it on their site with a Fix Kit and guide on how to replace the part in your Deck. This is why the fan sold out so quickly.
As of this time of writing, it is still available and you can buy the fan and kit for $30 over at iFixit! Heads up, it is only shipping to US addresses currently.
Are you going to bite the bullet and try to get this fan for your Deck? We would love to know! (And yes, I just purchased one as well).