When I have built wheels for myself I usually ordered DT spokes. My training bike is a Trek and has a broken spoke that I need to replace. Its spokes and nipples are average for an Asian bike. I can't find anywhere online to order an equivalent spoke. Most places only have straight gauge spokes by DT. I can buy one of those and install it, the only problem being a nipple that doesn't match the rest. Should I go ahead and go this route and just not worry about it?

The whole world uses the same standards for spokes, so a generic 2mm spoke from Japan, China or Taiwan will take the same UNC 2-56 thread nipple as DT, Sapim or Wheelsmith, etc.

So the nipple will only be slightly different cosmetically. Unfortunately the dimensions of the nipple's wrench flats aren't as standardized, so it may call for a different spoke wrench for optimal fit. That's why there are 3 different spoke wrenches for 14g spoke nipples.

If you want to, either for cosmetics, or to keep the nipple flats uniform, feel free to use the original nipple with your new spoke (assuming of course that it's still in OK condition).

So, just get the closest match you can find, making sure the length is right. Odds are you'll only find stainless steel, since not many places stock plated spokes for the aftermarket. Do try to replace butted with butted, or plain with plain, but even that isn't super critical except for cosmetics.

Who orders 1 generic spoke online?
Go to your LBS and spend a buck.
You have it in hand in a matter of minutes.

Bring the wheel Shop will put it in and true up the wheel for You .. for a few Bucks of labor time.

I did not know that nipples were interchangeable, I never tried it.
It's a 20 mile round trip across the city to a shop so I was trying for an easier way to get a spoke, but I just remembered that when my company moved our offices recently I found a small bike shop relatively close. I'll call them next week to see if they have a 14 ga of the right length.

Color? Are your spokes black or silver? Nonetheless, the best advice is to go local unless you're planning on rebuilding the whole wheel, or have something special like a bladed straight pull spoke.

They're ordinary straight gauge stainless steel, 14 ga., nothing fancy.