Hey man,
If you're just looking for card recommendations, a great place to start is EDHREC.
Volrath's page can be found here:
https://edhrec.com/commanders/volrath-the-shapestealerHas a load of excellent ideas on there.
The only thing with EDHREC is that if certain commanders have a few different strats then the synergies and recommendations can be a bit muddled.
Volrath has been built with counter synergies but also has been built as an infect build, so it's something to be aware of when looking.
Personally, I'd be a fan of a -1/-1 counter theme, hampering my opponents. As such, I'd chose things like
Hapatra, Vizier of Poisons and
Archfiend of Ifnir. The latter is an amazing way to get a counter on everything if you play into the discard / cycling side of things, but I have an unreasonable amount of love for that card so it might not appeal to you.
Once you've decided on a build style you want (-1/-1 counters, +1/+1 counters, infect etc.) let us know and we should be able to give a few more direct recommendations.
Also, if you could give us an idea on budget too, that'll help with our suggestions.
Good luck and happy brewing!