You don't say whether this is missing parts, defective parts or a modelling error that means you now require new parts. Depending on which it is you will need to take a different route
I hae no idea what country you're in, but in the UK the retailler of a sold item has a legal responsibility to sell something that is 'fit for purpose' so if a part is missing they should replace it. In practice of course they will probably ask the distributor. If you are in the UK then this is PocketBond http://www.btinternet.com/~pocketbond/
if you are not in the UK you will need to establish who your country's distributor is.
If you have made a modelling error they may sell you the sprues or sprue. Aternatively try contacting ebay seller Moon03tw if you need to buy parts. he has got me sprues in the past for extra projects rather than replacements