If it's not located in one the supported app sources listed on obtainium's github then I don't believe so - I couldn't find their github linked anywhere on the website. You could always download the apk directly from plebstr's website tho

The apkpure link works as source for obtainium if you trust it.

https://m.apkpure.com/plebstr-nostr-client/com.plebstr.client