Random question, what sort of shirts did enlisted men wear under their tunics!?
Pre-war/early war shirts were off white, later dyed a darker colour as an interim measure until proper field grey shirts were issued, if my memory serves me. The style was a half-button front with an unstiffened collar - although I believe a collarless 'grandad' style shirt was also issued - as well as a more traditional full button front shirt.
I'm pulling this off the top of my head while on the way back from work, so I'm happy to be corrected. 
'ABSIT OMEN'
Is there anywhere that sells them!? I did have a look before but prior to your message I didn't have a clue what I was looking for!
Okay, now I'm home I can have a look through my books: early war shirts were off-white and collar-less (this'll be where I was getting the 'grandad' shirt from), shortly replaced by a grey model with a collar; late war models were identical to the collared model previous, except they added pleated breast pockets. I was close!
Soldier of Fortune, Epic Militaria and Militaria.net all sell versions of the typical undershirt, and all are very pricey for what they are, in my opinion. 
[EDIT: Also, just noticed, only the Epic Militaria link I posted is the non-pocketed version. They may have the later war versions if you browse around, I just did a quick search.]
Really though, if it's just for airsoft use, you'd get away with any old grey shirt under the tunic. Save you a pretty penny, too. 
'ABSIT OMEN'
Grey collared shirt - end of.
You've got nothing to ein, zwei, drei, vier
If you can get one of these in your size:
https://www.varusteleka.fi/en/product/c ... sued/17273
they're pretty much a direct copy of a Wehrmacht model, shoulderboard loops and all, the trouble with Varusteleka being you have to pay for postage from Finland. Try and grab a few more goodies while you're they're to make it worthwhile
I've fired a bullet on every continent. Nearly hit someone, too.
