Yeah, 'fraid I've got to agree with VaKo here...
For educational purposes (or just screwing around with OSs) you not only don't want to look too hard for them, but you don't want to over-pay...
Besides this, it's very hard to get original copies of old MS OSs anyhow...
Also consider that it would be morally preferrable to steal than to pay for someone to steal, right? So, by that logic most of the copies on eBay are moral hotbeds :D
All this combined, I say hell, pirate it... I mean MS aren't exactly gonna extract any more money from the older OSs anyhow and they don't even support some of them. Go to it man!