Latrodectus hesperus is more commonly known as the Western Black Widow and as its name hints at, it is found in the Western part of North America (as seen in the green highlighted portion of the map below).

This is a map I made (using www.zeemaps.com) showing an approximation of where the L. hesperus
can be found in North America (the highlighted area). I used this map as a guide.

Latrodectus hesperus tend to be found in dry environments and low to the ground. They can be found in a variety of places including abandoned rodent holes in the ground, in cracks near vents with high insect traffic, and the crevices between rocks (Drees and Jackman 1999). Black widows are more common in agricultural settings than urban (Vetter et al. 2012).  According to Vetter et al. 2012, L. hesperus is being displaced from their natural habitats by Brown Widows, at least in southern California, which is beneficial for humans given the bite of the Brown Widow is far less detrimental than the bite of the Black Widow.

It has also become relatively common for black widow spiders to be pets. This is not necessarily recommended because of their venom, which can be quite painful if bit, but it is not usually lethal. If you're interested in that, here is a page that talks about their care.