Look, you could try ChipDocs.com, but though they don't HAVE the datasheet, you can fill up an application to receive the datasheet once they find it...
I didn't know, cause I used to get my "available" datasheets for free... Well... Let's say someone else used to get them, but I thought it's for free...
I don't know which producer does the ON588, but I found a site, **broken link removed**, I hope it helps you...
The site ON Semiconductor could contain your request, but I searched for "ON588", and guess what? Nothing found. Maybe I should have tried something else. Go there, maybe you'll find your way better than me...
Is your product by ANY means called 2N5883 or 2N5884 or 2N5885? If not, I don't think the site by itself could help you, but it may contain some useful links...
It could be anything, quite possibly it could be an 'in house' number (where the manufacturer of the unit asks for the number to be stamped on the transistor), so no datasheet would exist.