The First Step is to Confirm that the frame is not bent at the seatpost clamp area... Is the Slot in between the two clamp ears straight or does it taper at the top of the slot?

Next Step... Is the seat tube hole in the frame Actyually Round or is it ovalled? Measure it with the digital caliper to check.

Is it Round? then what dimesion did you read three or four times in a row?

straight slot? round hole in the frame? you now know your seat post OD...

Bent seatpost frame clamp? Oval Hole in frame? Contact a good Bike mechanic before you further damage the bike.

FYI.. 22.2mm seatposts are fairly common to many BMX bikes and older bikes like yours... 22.0 is available, but kinda rare... anything smaller is Hen's Teeth with gold fillings... Ebay shows Exactly ONE 21.8mm,as an example... there are Dozens of 22.2mm....
i'm thinking the frame clamp is crimped in too far.... and that your free Huffy may actually be a Murray in Huffy clothing... Murray used a 22.2mm Seat post sometimes... as per Sheldon Brown's Seatpost database and my personal experience.

Some folks make shims out of old pop cans by using heavy scissors to cut the cans... and well stocked Hardware Stores (ACE, not Home Despot) carry a stock of Shim Materials in various thicknesses... Aluminum is easy to work with and plenty adequate for the task...

check that clamp slot first, etc.