Shari Bus to Shiretoko

Hi,

I am planning to go to Shiretoko this June around 19th June.

But i can't decide where to base myself, either Abashiri or Shari / Utoro

I wanted to spend 3 nights in Abashiri because there seems to be much more that i can do there, like visiting lake Mashu and Kussharo.

But it seems that there's only transport available between shiretoko and Utoro only.

How do i find Shari bus schedule between Shari - Shiretoko, Shari - Utoro, Utoro-Shiretoko?

I am not sure what do you mean by "Shiretoko" since this is the name of whole area including Shari, Utoro and Shiretoko Five Lake, but is this what you want?

http://www.sharibus.co.jp/pdf/2017-04-P1.pdf

If you are visiting the Shiretoko Five Lakes, you might not want to miss the famous Kamuiwakkayu Falls.
IMHO, a rental car is the best option for exploring the area. In fact, in touring Hokkaido in general.

https://www.japan-guide.com/e/e6851.html