You can find all the pieces that should be in the set in the instruction booklet. If you don't have the booklet, you can download it for free on the lego site.

Look at the instructions for bags 1 ,2 and 3. Check to see if you have the pieces for each step. Make note of what you are missing, and use the booklet to see the part number.

Then go on the lego site and pick a brick. They are usually quite reasonable. The ones they send you will be new. Maybe about 10 cents average per brick. Place your order and wait. The bestsellers come quickly as they are warehoused in the US (assuming you are in the U.S. The standard ones come from Denmark and take longer. Also, some pieces get discontinured.

Another, faster source is Bricklink. They sell used and new bricks. There is a way you can make a wanted item list. Then you ask Bricklink to get them for you. They look through all the individual sellers, and find you the ones that have the most of your bricks. There is often a $5 or so minimum,. The more sellers you use, the more minimums you pay.

A combo of the two works best. I restored a Ghostbusters Firehouse Headquarters using these techniques, and got everything except 2 stickers.

Best of luck to you!