Buying Area and Regional passes in Japan

I usually buy my rail passes(voucher) in my home country and then exchange them for the actual pass when I arrive in Japan. I intend to visit the Kansai region in February 2024.
I would like to know if I could just buy the Kansai regional pass directly when I arrive in Japan without the need to buy the voucher in my home country. If yes, where can I buy these passes directly.

It depends on the pass! Some are on sale inside of Japan. Some are available online without paper vouchers, and others are available online with paper vouchers.

There are at least three passes I can think of when you mention "Kansai regional pass". See for purchase details below:

JR Kansai Area Pass:
https://www.japan-guide.com/e/e2361_03.html

JR Kansai Wide Are Pass:
https://www.japan-guide.com/e/e2361_08.html

Kansai Thru Pass:
https://www.japan-guide.com/e/e2357_005.html

JR West passes for example cannot be purchased at a counter in Japan from October the 1st.

https://www.westjr.co.jp/global/en/pdf/press_20230726.pdf

As said, you need to specify which pass. There are a lot of them, and not all are by JR.

BTW while it is true that JR West will unfortunately stop selling most of their regional passes at their ticket offices from Oct. 1st, they will still be selling a few like the Sanyo-Sanin Northern Kyushu Pass, plus you will still be able to buy their passes online.