Where in metro manila I would be able to purchase a macintosh operating system  

Well I guess you cannot purchase a Mac OS like windows are. If you want to use Mac OS you buy the whole MAC PC with the license OS with it. I think they are more pricey compared with other Desktops available. The only limitation that you would have is that there are very few softwares that are sold cheap. All of the softwares are with license and it will cost you a lot to invest in that program. performance wise MAC are good for graphical and multimedia applications. If you are used using Windows OS you will adjust a lot when dealing with MAC OS because they are very different with Windows in terms of GUI presentation and all of that. But eventually you may get the hang of it later, just be patient enough learn the loops in there.

If you are looking for a Linux OS they can be downloaded for free on the net. There are many variations of Linus OS, there are the UBUNTU, the most popular of all linux OS plus many more just search on it on the net. Like MAC OS you will have to relearn yourself in browsing the GUI platform. Although they are very much similar to Windows you can still feel the difference once you've successfully installed them to your PC. You can use that to your existing PC. No need to buy a new PC but you have to check because there are certian motherboard not designed for Linux and you have to download to change the BIOS at times. The limitation I see over Linux OS is that sometimes they are not as user friendly as windows is. If you are a gamer most of the games do not run good on Linux OS. Although they are not prone to be infected to viruses they have their downtimes at many times too and solving them is as complicated unless you know how to get through their command line then you are okay.