They are a knock off of the the old ATR (Applied Technologies & Reaserch) and Kenne Bell headers.
They do take a special downpipe, I believe RJC Racing and GBody parts will fit.
For that price ($<200) you get poor fitment, inferior materials and weld quality.
If you MUST have this style, check out GN1 Performance.com as they sell them.
The 15-20HP you gain is only if you put this style header on a 9 sec ride. Stock headers are good to low 10's.
Search Chinese headers for more info.