How do I get a os x lion installer? I originally purchased lion when it was released. I need to reinstall but have no way of doing that

Older installers have been disappearing from availability.

If the older release you're interested in—Lion 10.7, here—is still in your Mac App Store list of previous purchases, then you can usually download it there.

You can then run a web search for the sequence to build a bootable USB installer.

The older OS X releases did not have an officially-documented sequence, where the newer releases from El Capitan and later have the Apple-provided createinstallmedia tool to build a bootable kit from the Mac App Store kit download.

Or probably better for most applications as Lion is pretty old, download and build a bootable installer and use El Capitan, High Sierra, or whatever is the most recent supported by your particular Mac. Search for the Apple support note for details of the createinstallmedia tool, too.

One of the more common reasons for installing an older release is a corruption of some sort, and corruptions on older hardware can be due to a failing hard disk. Failing hard disks routinely show up as performance problems, app and system corruptions and hangs and crashes, and other degenerate behaviors.

Another reason can be preparing for sale or giving away, and loading El Capitan, High Sierra, Mojave or Catalina can work just as well for that.

If your drive is healthy and the Recovery Partition was not removed, you can boot into Recovery Mode by using Command + R or you can Option Boot and manually select the Recovery Partition (assuming you are currently running Lion). IIRC Lion has a severe unpatched vulnerability so I would suggest upgrading to a later version of OSX instead. We are not allowed to use Lion in our organization for this reason (Snow Leopard did not have the vulnerability and it was patched in Mountain Lion and later). I believe a 2007 iMac can go up to El Capitan OSX 10.11.

If you download a later version of OSX from the App Store, I would highly suggest you use Disk Utility to create a .dmg archive image of the OSX installer app before you upgrade since Apple will delete the installer app once the upgrade is complete. (You need to create folder and copy the installer to it so you can point Disk Utility to the folder holding the installer when creating the archive .dmg image). Then store it somewhere in case you ever need to use it again. I would even go so far as to create a bootable USB installer from it using instructions here and create an image of the USB installer which you can use later to easily "burn" it to a USB drive if you need to reinstall due to a failing or unbootable drive.