These days it it impossible to buy mens undershirts that stay white.  The cotton they are using is very poor quality.  I have been doing laundry for 35 years, have always bleached whites when needed, and use care with reading clothing labels.
I have purchased these shirts, they seem to stay white the longest, but I still need to bleach, and they still don't last any longer than 6 months before they are just unwearable.