Asexuality is something that people identify as. Just like some women identify as being a lesbian. It doesn't come on later in life. If you do some research on line you will see that there is a large community of people who identify as such. There are some threads here about asexual people who identify as poly. If you do a search for "asexuality" you will find them.
I suggest that you look for your nearest community and make some friends. There might be more bisexual women who are interested in starting a relationship with you more than lesbian women but be sure you know that anyone you involve yourself with in the poly community would want a relationship before sex... at least that is most common.
Maybe you should look towards the swingers community instead. Single women are welcome over single men or maybe your wife would go with you as support... there is no reason to look around your area... maybe a trip is in your future. Take the wife. Make a week of it.
I don't mean to make light, but it could be fun! And could be something that is very bonding.