If you want certain ones look at there teams and their twitter and facebook pages and look on the teams webpage. also look at their sponsors pages. Normally they will put info out of when and where they will be doing signings.
at Martinsville, you can go outside of turn 4 where the drivers motorcoach is located and you can drivers coming and going inside the fence in lot. On friday it would be the best day for that you would never know who you would get and depends on how their car is doing. alot of drivers sign at off the track venue so a place like jayski or even on their facebook page will tell you where they would be at. some driver will post on their trailer will post when they are signing but you would have to get there early in the morning to get a ticket. the tickets are usually giving out at like 7am and most people are there at like 5 am waiting in line.