You’ll see it if you search for it and in All, but it won’t be in your Local or Subscribe feed

…and it’s way cheaper than all of OPs suggestions other than the library

If you search by the full URL of the sub and make sure that Search is set to “All” it will show up eventually. If you then subscribe, your instance will then be federated and start to receive info from that community but it does take a bit of time to get.

If you join a medium-sized but weeks-old instance then it is likely to already have been federated and you won’t have much of a headache.