Hello Dean:
Sorry, but many, MANY of the parts for the Honda HR17 have long been discontinued from Honda, including the muffler, spark arrester kit, muffler cover, and muffler lid. You may have some luck trying to purchase an entire mower for spares, but they are quite rare too.
A few spares are available from private sellers eBay, but not many, and from what I can tell, most of them are still available from Honda direct as well.