Where can I get operating system iso's?

You can convert a physical disc to .iso with a free program like Daemon Tools lite. There are other programs out there (magiciso, poweriso, imgburn, but I’m not familiar with those).
So just pop your disk in your drive and use that tool, and it’ll let you create an iso to use for your virtual machine.

If you’re a member of MSDN (Microsoft developer’s network), you can download ISOs of certain OSes, depending on what’s setup to you (Through my school I can get 7, Vista and XP).
And most Linux distros are available as ISOs on their websites.

If the OSes you want to use are open source, you should be able to download iso files from the websites for each OS. If not, and if you have a license, you could probably make an iso file from your CD.
Here and here are instructions for making an ISO file using different freeware programs.

If you have access to the OS on a CD/DVD then you can use Alcohol 120% to create an .iso or .msi of the cd image. This software has a free trial which should be enough for you.

I might be able to get a windows XP installation disc, but I need an iso for mac OSX. I can’t get a hold of a mac installation disc.

Have you tried calling Apples customer service? If you have your license info, they might be able to help.

@llewis I don’t have any macs, just my PC.

@Sarcasm How do I get it to start. It doesn’t have drive e (my cd drive) available, and nothing happens when I insert a CD.

I put together a quick video for Daemon tools + VirtualBox. I hope it’s comprehensible, I know I do jump around a little bit. I chose to make it at 4am, which may have not been a great idea.
http://www.youtube.com/watch?v=JwK9jyXxCUQ

@XOIIO – but you said you wanted an iso for Mac OSX. Calling Apple would probably be the best way to get it. If you don’t have a license, it’s probably the only legal way to get it.

@sarcasm, I am having trouble with an existing iso, virtualbox says no bootable media but I downloaded the microsoft xp iso from the microsoft website. What doI do?

@XOIIO I’m not entirely sure if I’m just nitpicking your word choice, or if I’m actually correcting a problem. I hope it’s the latter.
But you don’t actually download ISOs from the Microsoft website (at least, I didn’t when I got XP and 7). Instead, you get their downloader program. The system is kind of analogous to torrents, where you go to a website and download the .torrent file for the thing you want, which is then in turn used to download the thing you want. So you’ll have to run that downloader tool, then you’ll have the ISO.
After that, just open up VirtualBox and go to File -> Virtual Media Manager -> Add and find the ISO that you got.

It turns out the ISO (which I did download from microsoft) is for windows xp service packs 3, but it isn’t bootable, it’s just an upgrade or something. I know a couple people I might be able to get windows XP installation discs from, and I have 3 programs I can use to make an ISO with.