I'm happy to hear you've discovered something wonderful that you would like to have in your life and I'm sorry to hear that you and your wife have lost the magic in your marriage.
There's a general consensus in the poly world that your primary relationship needs to be strong and solid before you start adding other people to your life. At this point, even if your wife did give her permission, it would most likely only drive a deeper wedge between the two of you.
Right now, it would be good to focus on your marriage, either fixing it or deciding that it can't be fixed. If you decide to pursue a polyamorous lifestyle, that can come after you take care of the immediate situation. Getting together with another woman will do nothing to fix the immediate situation.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker