As you already figured out, there's SSHelper hosting its own
.apk. But you might also wish to check with F-Droid, and find out there are even more SSHd's to download without using Google Play Store.
Edit: As it turns out, I wasn't aware F-Droid also features forums, and the above are only forum links. To compensate: You can also find some SSH Server apps on 1mobile, e.g. SSH Server. The website allows you to simply download the
.apk from your browser, no login required. Other than with F-Droid, I'm not sure concerning their reputation, though.
Apart from that: In my experience, it's always worth checking the resp. dev's website, where they sometimes host their own
.apk files for download. If not, asking them helped me as well :)