I've posted the following on a couple of the metal shaping sites, when a similar question has come up.
If you look at each gage and the tolerances, you will find that there is some overlap between the gages. The info below is out of the "Machinery's Handbook, 21st edition, page 467" for sheet metal. When you call your metal dealer, ask them if they don't have 19 gage for the thickness of 18 gage. It might measure close enough to the upper tolerance for 19 gage to work for you. Even it is a little more, it might not be that much of a difference. Most of us would rather work with a little extra instead of less.
Several us have been talking about a metal purchase on the west coast, you would be surprised on some of the metal thicknesses we have been quoted. The only way that you will know for sure is to mic it and it will vary some.
The tolerance for these gages is +/- .004, this was from the table on page 467 for Cold Rolled Alloy Steel (for coils and cut lengths) between 40 and 48 inches wide and between .0314 and .0508 thick.