I have read a couple of fiction novels, based on the renowned manuscript. But is an English translation of the actual Arabic text available online?
There are some English translations. Unfortunately it appears most of the manuscript was found fairly recently, which means no translations are Public Domain (and probably never will be), which means Project Gutenberg isn't the help here that it usually would be.
So I don't believe it is, or (barring some help from Google books or a kind Gutenberg volunteer who knows Medieval Arabic) ever will be officially online somewhere.
I did find a copy online with some judicious web searching. I'm not going to link it here though, because firstly I'm unsure of its copyright status, and secondly I found it as a ".DOC" file on someone's personal university web page. If that URL gets published far and wide, it will probably quickly disappear (defeating the entire point of posting it here).
What I will tell you though is that, upon realizing its usual name is "Risala", a web search for "The Risalah of Ibn Fadlan" found it on the first page of hits. It was on an Indiana.edu website, in [DOC] format.
Here's an English translation of earlier (1939) Russian translation.
There is an up-to-date English translation of Ibn Fadlan, advertised here:
It is a paperback (Penguin Classics) and very cheap. Or you could find it in a public library.