Well, you need to know which version your ROM is on the A500+. I believe they originally shipped with the 1.3 ROM, but it was fairly common for users or dealers to change them up to 2.04. You need to run a Workbench that matches the first number of your ROM. (So, in other words, the 1.3 ROM can run Workbench 1.1, 1.2, or 1.3... while the 2.04 ROM can run Workbench 2.04, 2.05, or 2.1.)
Unfortunatly, you're going to have to buy it, or find a friend who has a configured Amiga. Normal PCs can't write to Amiga disks, so even if you find a disk image, you have no way to get it to the Amiga.... :-(