They are made as a universal item in that they will fit D4, D5 or thriple 3 etc. This refers to the siding exposure ie: D5 means double five. Mid america is a manufacturer that makes them in most colors to match the siding color. I purchase mine from a siding wholesaler. Most phone books will have a listing for suppliers under siding in the yellow pages. Some of the big box stores carry them in limited colors.
Keep in mind that they are installed BEFORE the siding then the siding is notched to fit around it and then a trim ring/square is installed to cover the rough edges. Be sure to allow a gap so the siding can expand and contract without buckling.