After leaving Vault 111, his house contains a floor safe beneath a dresser. A terminal belonging to a chem dealer in Sanctuary Hills states that Mr. Russell purchased X-cell at some point, although the dealer suspects that he may have used it on their dogs.
↑Ledger terminal Russell: They still owe me for that last X-Cell delivery. Occurred to me - was it even for them? Because, so help me God, if I find out they're juicing up those dogs, we're going to have words about more than the money. But we're probably beyond words anyway. I know they have a floor safe in there - I've seen it. Master bedroom, under the dresser. Can't be that tough to crack mate.