This is a very common spark plug. You shouldn't have any problems finding it at any automotive parts store.
You will need a Champion RJ18YC/130-435 spark plug. Gap it to .30. You can buy them here: http://www.jsesc.com/
Two compatible spark plugs for a Troybilt weed whacker that currently has a spark plug with a model number of 794 00055 is the Champion E3.16 or the Champion Spark plug RDJ7J. Both of these spark plugs will give your weed whacker the starting power needed in a replacement. You can buy them here http://www.mymowerparts.com/partdetail/1060/1042/10783.php
Depending on the model, it could differ, but the most common would be the champion CJ8 spark plug, or similar, double check at your parts store before you buy.
Champion CJ7Y or NKG BPM7A or Megafire SE-Y7C or Bosh WS7F
One of the things that a mechanic could be told to buy when working with spark plug wires could be a spark plug gap wire gauge. Another thing could be spark plug wire pullers.
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You buy a spark plug tube removal tool.. http://www.spendoo.com/1654-buy-pens/4889887-toyota-spark-plug-tube-remover-installer.htm
with a spark plug gap tool buy one for 3 bucks at auto parts store
Spark plug gap is 0.70 to 0.80. they come standard when you buy it.
Buy a plug wrench.
On my 1992 Tercel the gap is .044 inches. When you buy plugs, the manual (at least the Champion manual) should also have gap specs for the car.
Just like any other spark plug you just need to find the size of the plug and buy it.