I've found that if they don't show up in search, you can go directly to the url and subscribe there. After that it will actually show up in search here.
For example, if you find [email protected] at browse.feddit.de, but it doesn't appear when you search communities for whatevercommunity, then you can go to the url https://lemmy.perthchat.org/c/[email protected] and subscribe there.
This has been a bit hit and miss for me though. I'm pretty sure it's supposed to work, but sometimes you'll get a 404. My guess is that it's because the initial request the perthchat instance makes to lemmy.ml might pull down a lot of data, and if lemmy.ml is under load it might drop the request.