How can I thicken Acid Stain myself?
Quick Google search turns up Modello Gel-lo


  Modello Gel-lo is a thickening agent for  liquid coloring mediums such
  as acid and water-based stains for concrete. The addition of Modello
  Gel-lo allows for more controlled color application and helps to
  reduce "wicking" of the stains under the edges of the Modello
  patterns.


And Stain Mule™.


  Stain Mule will thicken both acid stains and water based colorants.
  When color placement control is necessary, the Mule is your product.