Hi there
The website is www.savana.com.mk but even though the website is not very informative it is an agency with very good reputation.
If you want to contact them you should call them because i don't think they use email very often.
I have never been to Popova Shapka so therefore I don't know anything about ski rental possibilities :(. You should ask the agency about that. But if I were you I would just bring my own skis because even if you can rent some the rentals will be probably very worn out.
I know there is some hotels on Popova Shapka and you can probably book one through the agency also. Popova Shapka is really close by and it shouldn't be a problem to commute to there every day. It takes about an hour to get there.
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