Where are you located?? Have you done a search via
www.petfinder.com ? What about adopting from a rescue organization? I did a quick search on PetFinder and found 83 female Rotties (some were mixes) in NY state...
As for adding another male to the house, I would not recommend it unless ALL avenues for getting a female have been exhausted. Two males of close age will sometimes fight for dominance and it ain't a pretty sight. NOT saying it WILL happen, but opposite sex dogs are usually a better match.
Sending hugs to you and your dog to help soothe the loss of your friend & companion rottie. So sad when they leave us. :(