Can you buy tickets for these sessions, if you are unable to attend the race itself?

Anyone holding a ticket can watch qualifying and practice. Some tracks have free admission if it is just an open practice session like they've done at Lowe is Speedway this year.

if practice and/or qualifying are on the day of the nationwide race, which they usually are, you need to buy a ticket to the nationwide race to get in and watch it.

yes all you have to do is buy the ticket for pole
Example here at Chicagoland it is $15 for Pole day you get to watch pratice sessions for both and qual for Sprint cup..except this year qual was rained out
i take my niece and since she was under the age of 12 she was free
and here you can leave and come back as many times as you want..

It is different at each track. Some of the sessions are before or after the Truck or Nationwide race and if so you usually need a ticket. Others are free so check your local track, they will show it on their schedule.

anyone can