You may have a sharp object that is stuck in the tire, so it puts a small hole in each new tube. Check the tire carefully inside and out.
Most tubes lose a bit of air gradually, but not much in just 1 day.
You can patch the tubes. Buy a patch kit or 2 from the bike shop. Put a little air in them, then fill a pot or sink with water & look for the bubbles. Dry it well & follow the patch instructions.
Most people have good tubes with them when they ride for on-the-road flats, and then patch later when they get home.