How much height do you need?
For years, I have used a block of wood (2 x 6, by about 12 inches long, just a scrap piece of wood that I found), that lifts mine about 1.5 inches.
Whatever you use needs to sit square on your desktop, so your iMac doesn't move around. I have some handy shelf protector (that nobbly vinyl stuff that you can find anywhere that kitchen accessories are sold) that I have the iMac sit on, and it doesn't move unless I want it to move.
You can always paint a block of wood some nice matching color, or contrast for beauty, I suppose. Mine is just a piece of wood, and looks the part