I have a Nighthawk Mens 18-Speed Mountain Bike. When it comes time to replace the disc brake pads, I saw that Huffy sells the whole brake caliper for $8.
It seems like it would be cheaper to just replace the pads, but I can not find them.
Chances are the calipers are compatible with with something like BB3 pads. Problem is finding some will be more expensive than buying new calipers. You could look at pictures and 'take a punt' buying off the internet, or go to local bike shop that can match to your calipers with new pads - but that will cost at least twice as much.
I suggest spending a bit more and upgrading to a known brand of caliper, so when it comes time to replace pads you don't need to go though the exercise again.
With Huffy bikes, it's actually more likely that you will not find a compatible pad from a 3rd-party distributor. This is unfortunate, because a nice pad by a reputable manufacturer can make a huge difference. However, it's still pretty easy to replace these. You'll need to contact Pacific Cycles. There ought to be a sticker on your bike, probably down on the lowest part of the tubes between the two pedals. This will have the 1-800 number as well as a date code. I've had to order parts for customers several times and I find their customer service surprisingly adept: simply tell them the code and what you need and they will ship out the part to you. The one time I can remember replacing pads on a Huffy, they were about $20, so it's up to you if that's worth it. Just make sure the new caliper actually comes with pads if you plan to go that route.