I checked Home Depot and Lowes and didn't see anything that would work. I can try the supply house. Does anyone know what size fittings the smaller and larger units use?
Don't waste your time going to home improvement stores for welding supplies. Go to whatever welding supplier you get your cylinder gas from.
Standard oxyacetylene hose fittings are "A" size (small) and "B" size (larger). B is used for most things now. A used to be common for small torches, but is pretty much only used for Prest-O-Lite air-acetylene (plumber's) torches now.
A to B (and vice-versa) adapters are readily available from any real welding equipment and cylinder gas supplier.
Take your Harbor Freight torch with you to make sure it fits a standard B fitting, because the last time somebody had a (brand new in the package) Harbor Freight welding hose around here it looked like it had B fittings, but they wouldn't screw onto the B fittings on a US-made torch and regulators.