Startech racks on Amazon are pretty decently priced. I had seen a 42u for around $200 awhile back.
https://www.amazon.com/dp/B07DKJ26SK/ref=cm_sw_r_cp_apa_fabc_UoU0FbFAHXSJ5
Otherwise you can scour the internet for auctions near you. Usually when they decommission a data center or an large company upgrades you can get racks, servers, etc relatively cheap. You really don't see racks during upgrade cycles though. Sometimes I luck out on Ebay.
I got it for 200 dollars AUD from Amazon but it was the last one. here you go
Amazon is a good place to start, some like this one might work. https://www.amazon.com/Frame-Server-Enclosure-Adjustable-Depth/dp/B07DKJ26SK
I just got one about a week ago, can't beat the price. Build quality seems flimsy at first but after I racked my servers it feels solid.
Looks like it's about $190 usd as of now
Thanks! I'm using this 27u rack. It was the cheapest adjustable depth rack I could find that had enough space for my stuff :)