There is particularly one lab coat through Amazon that most Okabe cosplayers (myself included) get. https://www.amazon.com/gp/product/B002B5XK54/ This is below kneecap height on a 6'4" 245# person. 50L is about it. It's still not quite "that" anime or costuming ankle length, but real lab coats typically avoid length for fire and safety reasons. Beyond this, you're looking at commissions, making your own, or a costume (ie halloween) lab coat.

I might also suggest these in Bark color: https://www.amazon.com/gp/product/B002AQRPMI They are somewhat expensive, but are pretty durable and comfortable. They also are the closest match I could find to an average of about 50 different color samples of Okabe's pants over the course of the anime. I've bought several more in the same style after using them for cosplay.