You don't say which operating system version you are using or which version of iMovie you want.
I can read your post from across the room. My computer monitor is only so large and I need the space for other things too.
Old versions you will have to find on eBay or used software dealers, or ones with old stock (sometimes Amazon).
If you have the original discs that came with the computer, then the iLife applications can be restored from the Optional Installs (or Pre-bundled Software) folder. Double-click on the installer in the folder. Start by inserting the first DVD, Disc 1.
If you don't have those discs, then you can purchase an iLife boxed set by looking on eBay, Amazon.com, or searching Google.