You probably can't buy AmigaOS 3.9 any more.
For a time, various Amiga retailers had stock of the physical AmigaOS 3.9 CD-ROM - the only format it was available in - but it would seem to have sold out.
The legal owner of 3.9 could probably initiate a new production run of CD-ROMs, or offer it for download, if they wanted to. However the legal owner is not easy to determine. It was developed by Haage & Partner, a company which no longer exists. Even if their successor could be identified, Haage & Partner were developing it under license so they may not have the rights to make another production run. The licensor also no longer exists, as its corporate entity has changed hands several time in that time frame and subsequently.
If you want a legal copy of AmigaOS 3.9, your best chance is to watch the second-hand marketplaces.
However, AmigaOS 3.2.2 (which you are currently running) is actually more recent than 3.9, so there's probably no need for you to seek out 3.9 unless you are interested in historical versions of AmigaOS. See What's the most recent version of AmigaOS?