If you can't get a rack [and if you live in any populated area and have a pickup truck you can probably find free if you do all the heavy lifting] the next best option is shelves. Heavy duty shelves enough for each server to sit on its own. Make sure they are actually rated for the weight of the servers. Servers are heavy. Make sure they are bolted to the wall or floor so they don't tip and crush someone.
Stacking them all up can cause all sorts of problems. Vibrations. Air flow issues. Grounding issues. Cabling issues. And safety hazards - they will slide around fairly easily. They may fall on your foot.
Putting plywood or foam between them may help vibrations but make other issues worse.