I encourage you to check out an interesting article by Geoff Nicholson from the Los Angeles Review of Books, in which he seeks out the actual locations where a number of books take place, including I Am Legend.
I, and others, have made attempts to find the number of the house where Matheson lived on Cimarron Street, and it seems he simply doesn’t remember. Cimarron is a long street, stretching from above the 10 to well below the 105, and there’s also a Cimarron Avenue and a Cimarron Way in Gardena, which actually fit better with the geography of the novel, and although I enjoy an aimless drift more than most, a dozen-mile expedition to an unspecified address in Gardena was too nebulous even for me.
That said, Nicholson was able to retrace the steps of Charlton Heston from a number of locations used in the filming of The Omega Man. You can read the whole article here.