I use the Wilton slicer (the big one) and it works just fine for me, though the height guages are limited. You can also try looking in a thrift store for a long serrated bread knife or just buy one in the store, I would think Walmart would carry something like this.
You did ask for the *BEST* way to torte a cake. The best way is an Agbay.
Second best way:
Using a long serrated knife and the edges of the pan as a guide, cut off the dome.
Remove the cake from the pan.
Measure height of cake and note the halfway measurement.
Add enough cardboards into the pan to raise the cake to half it's height.
Put the cake back inthe pan, check the height measurement one more time, adjusting the # of cardboards if necessary.
Using the long serrated knife and the edges of the pan as a guide, torte.
The height adjustments are as infinite as the height of the cardboard.
You can buy a long serrated knife at ANY department store with a housewares department, knife store, Target, Walmart, etc. They are labeled "Bread Knife."
999988294849998898th best way:
Use either Wilton slicer.
Fishing line or unflavoured dental floss also works if you're going to use the pan method as Leah suggested. Works very well!
OMG....Leahs, you are a brilliant!! Great way to torte and have it STRAIGHT!!! Awesome 
The floss is nice as it can be any length--bread knives are not long enough.
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