If you know how a shirt fits and wears, it makes it a whole lot easier.
A while back I sourced a ton of blank shirts from wholesalers, mainly attached to the printing industry, but available to the public. Might have bought a few too many, but I will never run out and not worried if anything happens to one as they worked out so cheap per shirt, only a couple of $$ each really. Things might have changed since then, but that is the best way if you want more than just a few.
Gildan has a few options, as mentioned, but the 5000 series is my fav - fairly short overall compared to some brands but a really good tee for wearing around and fits well under everything else too.
AAA or Alstyle is the brand that all the old skate tees were done on back in the day - thicker and more long lasting, but good fit and not overly baggy / long for the sizes.
Fruit of the Loom - FOTL - were also good shirts when I had them.
There are a few other brands out there, but look up blank wholesale and you might find a bargain or two.