the suggestions you got were bad. you should not buy a rick owens t-shirt or ones from COS - what makes those tshirts good is the opposite of what you're looking for - they're sheerish, and they're frequently made of blends so while they drape well, they're often see through. rick owens tshirts are also $500, and why pay $100 to proxy a tshirt? if you want a cheap serviceable decent fitting tshirt look at club monaco or jcrew. you're a pretty regular shaped guy, so most random mall brands will work fine. if you want to spend more money look at LVC, RRL, wings + horns or japanese denim brand (ironheart, loopwheeler). the thing is. though, if your purpose is to wear it under a knit, it doesn't matter what it really looks like, so why spend $100 when $10 will work.
what irritated me about your post is that it reveals one of the most frustrating parts of SW&D - people coming in here and saying 'dress me!'. try some stuff out. experiment. that's what makes streetwear fun - it's much less paint by numbers than CM.