You can hire a Spider Run; the contractor will complete the quest for you, allowing you to charm a widow.
If you can run it yourself, bring a hero to charm the spider.
Black Widows in the wild are always level 20. If you want a particular evolution, you have two choices:
Make multiple runs, until you find one with the desired evolution.
Charm the first spider you find, release it at the menagerie, and then death level an L5 widow.
The widow that spawns near Ashlyn Spiderfriend cannot be charmed by players or heroes; she uses a version of Charm Animal without an activation time, which means there is no way to prevent her from charming it first.