Hi Martin, welcome to the forum.
I spent six years in Yogya and failed to find IPSI's address too, so don't blame yourself
anyway, you can find KONI's (Indonesian National Sport Committee) office next to Kantor Pos Besar Yogyakarta ... if you can find anybody there (the place look deserted to me), they may be able to tell you anything worthy (to be honest, I doubt it... so just drop by only if you happen to be in the area).
A visit to silat sport may give you a good start, usually they come from or know traditional silat. I'd suggest Gelanggang Mahasiswa UGM or Universitas Negeri (IKIP) in the afternoon.
Where do you stay Yogya?
@Kak Kuning, Mas Bram lives in Madiun... but he knows Yogya very well. We haven't seen him around for long time here, kinda miss him actually.