Chromates and Dichromate compounds, when mixed with strong acids (such as sulfuric) form chromic acid in solution. These are much easier to obtain than chromic acid.
If you needed one pound (453.6g, 4.536 moles) of Chromic acid (CrO3), substitute 675.9g of Sodium Dichromate Dihydrate or 667.2g of Potassium Dichromate with 224.1g of Concentrated Sulfuric acid.
This is almost exactly 1 1/2 pounds of either dichromate and 1/2 pound of sulfuric acid. If you have additional sulfuric or nitric acid in the mix, perfection when measuring is probably not necessary, but helpful.