Would you be kind enough to offer me some insight as to where to search for discontinued furniture?
Even if they don't it is possible the dealer to get in touch with the manufacturer regarding the missing pieces. That might work.
However, it is not always possible to get these pieces, especially if it has been a while since the pieces were discontinued. Manufacturers often sell these pieces to closeout stores.
You may want to look at furniture outlets and clearance centers. But that can be very time consuming, and the outcome may be uncertain. They might not necessarily have the pieces you are looking for.
Try scouting local garage sales and estate sales if you are willing to buy used furniture. If it was a popular line you may have some luck. Try posting online on Craigslist.org or e-Bay. When doing that you may want to specify which area you live in, unless you are willing to pay huge shipping fees.
Your best bet is to get in touch with a local dealer who carries the brand you are looking for. Try a few stores to find someone who will work with you.