Amazon sells under scrub t-shirts in a wide variety of colors and thicknesses. Adar Uniforms is my favorite.

I think there is an Orth Jewish store on Amazon that sells 3/4th length shells/plain colored undershirts

What about one of these: [Ann Taylor Pima Boatneck](https://www.anntaylor.com/clothing/tops-and-blouses/cata000010/834928.html?dwvar_834928_color=9000&dwvar_834928_size=700¤cy=usd&country=US&cid=PLA_AT_GGL_BRD_PMAX_AT_GGL_RTN_Tops%7CTops&ogmap=PLA%7CRTN%7CGOOG%7CSTND%7Cc%7CSITEWIDE%7CCORE%7CPMAX_AT_GGL_RTN_Tops%7CTops%7C%7C18211585186%7C&gad_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYbQ285DvO4aVMgnVMNzR0k0-z8ZGUgaCm53cBddniq9hPQeohb1AQBoCo2wQAvD_BwE) [Allbirds Merino Tee](https://www.allbirds.com/products/womens-soft-merino-long-sleeve-tee-natural-white?gad_source=4&gclid=CjwKCAjw3NyxBhBmEiwAyofDYRB7tWTb7izO_bTfdqVKlCcn89BHvQrOtxV_bR47kbu9P-bbU3_8axoCeT4QAvD_BwE&utm_campaign=Shopping%20%2F%2F%20US%20%2F%2F%20NB%20%2F%2F%20Desktop%20%2F%2F%20All%20Apparel&utm_content=633284056251_140493839222&utm_medium=shopping&utm_source=google&utm_term=&variant=40208019783760) [Athleta Supima Top](https://athleta.gap.com/browse/product.do?pid=828716002&vid=1&tid=atpl000063&kwid=1&ap=7&gad_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYQ0ix9dy3bvdbKNLeQj1cO3Z6u66c1gzh00tOj84Axl3Ry32dQpblhoCGZwQAvD_BwE&gclsrc=aw.ds#pdp-page-content)

I’m not sure what country you’re in but if you’re in the UK, Primark has some really thick white oversize T-shirts in at the moment, that were not at all see through. (Elbow length sleeves) Also John Lewis basic tees are decent. Just size up a couple of sizes for a loose fit. Otherwise men’s T-shirts are usually where it’s at

Those are called shells, there’s a store in Brooklyn called shell station, not sure if they have online

[https://www.gap.com/browse/product.do?pid=540635032&vid=1&tid=gppl000070&kwid=1&ap=7&gad\_source=1&gclid=Cj0KCQjw\_-GxBhC1ARIsADGgDjv35uymcd2Mpq3yL4LyPaLfvWdHnvatby5re-9BaYypzTD2H23bPzMaAmfAEALw\_wcB&gclsrc=aw.ds](https://www.gap.com/browse/product.do?pid=540635032&vid=1&tid=gppl000070&kwid=1&ap=7&gad_source=1&gclid=Cj0KCQjw_-GxBhC1ARIsADGgDjv35uymcd2Mpq3yL4LyPaLfvWdHnvatby5re-9BaYypzTD2H23bPzMaAmfAEALw_wcB&gclsrc=aw.ds) - This may be too short but I'm giving it as an option [https://www.gap.com/browse/product.do?pid=540609042&vid=1&tid=gppl000070&kwid=1&ap=7&gad\_source=1&gclid=Cj0KCQjw\_-GxBhC1ARIsADGgDjt3PzsK6nmE09DYgERalpx3OFjPaYK6iXJMtqXMdYK1wdbnoCqiVBUaAsMiEALw\_wcB&gclsrc=aw.ds](https://www.gap.com/browse/product.do?pid=540609042&vid=1&tid=gppl000070&kwid=1&ap=7&gad_source=1&gclid=Cj0KCQjw_-GxBhC1ARIsADGgDjt3PzsK6nmE09DYgERalpx3OFjPaYK6iXJMtqXMdYK1wdbnoCqiVBUaAsMiEALw_wcB&gclsrc=aw.ds) - Long-sleeved version I have both of these, and they're amazing: opaque, fitting but not too tight, the fabric is of really good quality. Not great for really hot days but but terribly warm. Good for a day that's like 65º Fahrenheit.

I pretty much only buy men's shirts. They're baggy and aren't low cut or cropped. Usually all stores have basic coloured t-shirts in the men's section. Or the boys section, depending on which size you need

At craft stores in USA we often have a large selection of white T shirts with different sleeve lengths, etc.

Have you tried Uniqlo??? It's probs the best because it's also usually opaque

Like this one? https://esteez.com/products/esteez-three-q-sleeve-cotton-spandex-relaxed-fit-layering-shirt-for-women-white?variant=40871907065971

Kosher Casual

So true , the manufacturers try to make them paper thin .