The highest qualliy I know you can find here, I doubt you'll be able to find better qualliy anywhere else unless you decide to buy physical copy.
Ye, it's not a website, but it's in best possible quallity and has all the pages that are missing on pretty much all manga sites.
https://www.amazon.com/gp/product/1421577445/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
this then the last 4 chapters that arent collected into a volume
This is a very in-depth book about the actual Musashi as well as his writings (including the famous Book of Five Rings) and is based upon official records and narratives from people of that time period (https://www.amazon.com/dp/B00K0N6F3S).
In the case of the Musashi-Kojiro duel, the author posits that Musashi did arrive late, stunned his opponent with either a wooden oar or a large wooden sword, and then left quickly to avoid being ambushed. The stunned Kojiro was then killed by some of Musashi's disciples who were in hiding. There was a subsequent outcry from this and Musashi was escorted in safety to his father's house in another province by a company of musketeers employed by the Hosokawa clan who had put the duel together in order to remove Kojiro from a key position in the province.
The character on Eiji Yoshikawa's fictional account of Musashi is very loosely-based upon the more complex historical figure although both the fictional and the historical Musashi figures meet is in the fact that Musashi was truly a skilled (but careful fighter) and was indeed a skilled artist.
Great vid, I agree with many of the points about the Samurai movies too (at least with the first one). If you ever end up doing a part 2 I have a few more suggestions of his appearances in fiction too - Obviously, the most well-known one is in Baki, then you have a young Musashi in Tenkaichi, and then there is something that seems to be not as well known by people and it's the graphic novel of "The book of five rings" - https://www.amazon.com/Book-Five-Rings-Graphic-Novel/dp/1611800129 (I think this is the one, I did not want to link a pirated site)
As far as I know MangaLife and Mangasee have the best quallity, but I found out by accident that 2 pages of one chapter are missing (not much happened, most people probably wouldn't even realize it, I just happened to find it by accident, but still...). So if you don't mind that, it's a good option. If you are reading on phone you can download app called Tachiyomi and download extension like MangaLife or any other manga source if u want to download anime and read it offline.
(Not only on Mangalife or Mangasee, even on other manga sites, bigger or smaller, exact same 2 pages were always missing, I checked myself)
If you don't mind torrenting you can download all volumes and chapters here (no pages are missing as far as I know). However, you will need a cbz reader, on PC I recommend YAC Reader, on phone ComicScreen/ComicViewer should work.
https://www.amazon.com.au/Vagabond-Vol-VIZBIG-Takehiko-Inoue/dp/1421522802/ref=mp_s_a_1_1?dchild=1&keywords=vagabond+6&qid=1609283785&sr=8-1 that should work. I went to some US stores and I couldn’t find any. I guess you might need to start shopping more internationally unfortunately if it’s starting to get hard to find.