Fedi.Tips@FediTips@social.growyourown.services
p.s. This doesn't work for 100% of Ghost sites, but it seems to work for most of them. It depends on whether they have Fediverse compatibility switched on.
p.p.s. I know the discovery process above is fiddly, which is why I've started manually curating good quality Ghost sites that are Fedi compatible on fedi.directory along with their Fediverse addresses. You can browse the ones listed so far at https://fedi.directory/tag/Ghost and just paste the addresses straight into your Mastodon etc search box.
Fedi.Tips@FediTips@social.growyourown.services
p.p.p.s. If you are finding this all a bit complicated and you just want to click on an account and follow it, you can see lots of good Ghost accounts listed in the latest post thread over at @FediFollows@social.growyourown.services
(Don't worry if an account looks blank, this is normal. This is caused by being the first person from your server to follow an account, but new posts by the account will show up as they are published.)