What easily accessible biological material will live the longest?

I'm an artist and have no scientific knowledge whatsoever (please excuse my basic language here). I require a simple biological sample e.g. like blood, saliva etc, that when placed under a microscope, it will stay 'alive' for at least 24 hours (longer the better). I tried using my blood and unfortunately it dried within a few minutes. The aim of my project is to have live organisms moving around when viewed under the microscope. Also, are there any particular liquids or techniques I could use to preserve the sample longer? I'm limited to only gl slides and slip covers. Also, please don't say sperm because its not an option. Many thanks for helping.