Your local Home Depot or Lowes has a good small hardware selection with many specialty types and metric. I have a lumber yard and a hardware store that both carry some metric and my town is less than 2000 pop. And any TrueValue or Ace hardware affiliate should have some stock that they sell in onesies. These stores generally have thread gauges so you can determine the thread and diameter of the items you are trying to match or at least an assortment of nuts you can match it to. A lot of screws that come with HSFs, fan adapters, etc. are not metric, they are English like number 4 or 6-32. Metric generally has finer threads than English/American sizes. Good luck. McMaster-Carr
www.mcmaster.com has about any type of screw, nut, what have you under the sun, but you'll likely have to buy many more of each size than you need, always best to try locally for onesies/twosies if possible.
And there are really no american equivalents to metric - they will either be a bit too small or a bit too big. The Ultra Kaze has open corners so you could use the 25mm fan screws that came with it thru the bottom ears of the fan (shim out with washers or tubular spacers if necessary) and mount the guard into the holes in the top ears with standard fan screws. Always a work-around if the old gray matter is functioning adequately.
.bh.