You can go to Dewalt's service site at http://servicenet.dewalt.com and search for your model number in the find parts search box to get the manual and/or parts breakdown. Another good site for a parts breakdown is www.ereplacementparts.com. You can order parts from either site or get part numbers and search the web for better prices.
Dewalt doesn't have a DC987 chain saw. They do have a cordless drill with that model number and you can find a good parts diagram at www.ereplacementparts.com. Put the model number in the search by block and you'll get a good breakdown and a list of what parts are still available. NOTE: Shop around once you get the part number, ereplacementparts isn't always the cheapest place to get parts.