I wanted to buy a specific subclass from Tasha's Cauldron of Everything but I cant seem to find the individual subclasses it used to offer. I dont want to buy the whole book for it so it'll be nice if someone could tell me how I can buy specific subclasses

Piecemeal / a la carte purchasing is no longer supported.

Where's the Echo Knight subclass?

Quote from Uncle_Guam >>

Where's the Echo Knight subclass?

 
It's part of the Explorer's Guide to Wildemount book. 

You will have access if you have either of these:

• have bought the Explorer's Guide to Wildemount book
• bought the subclass from marketplace prior to the Marketplace change on April 30th.
• are in a campaign with Content Sharing enabled by somebody with a Master tier subscription where a player has purchased the subclass or book.

If you have access then you will need to enable Critical Role content on the Home tab of the Character Builder.

Of course it's not.  Why would WotC want to support a consumer friendly feature.

They should create an item thats just every subclass in the expansions.   Why have players buy monsters and maps to play a subclass.   Grouping them all into one purchase will still let them get a chunk oh money, but now it is at least something players can use.