Every toolbox either has or SHOULD have an adjustable wrench. Get yours here! If you just need a basic one for small jobs around the house, you can get a lower end wrench made by Elenco or Velleman. You can even find a couple higher end adjustable wrenches at an affordable price made by Gedore.
Otherwise, if you need an adjustable wrench for work or you just need something truly reliable with a long work life, you can grab one made by Hazet, Ideal, Klein Tools, Knipex, and Wiha. You can even get a Wiha insulated adjustable wrench if you need one.