Ye got three options:
1. Buy pieces directly from LEGO. LEGO has a thing on their website where you can pick and choose individual pieces and purchase them. You can access Pick a Brick here. Be warned however they are kinda pricey.
2. If you don't mind buying used LEGO, check out Bricklink. Bascially people set up their own "stores" within Bricklink and they sell individual LEGO pieces. Prices are generally much cheaper (this will vary from store to store) but if you need a variety of pieces you may have to purchase from multiple stores.
3. Go the cheap route and make them digitally! LEGO offers a program called LEGO Digital Designer. With this you can make your models virtually and it won't cost you a thing. In fact they even have an option where you can order the pieces from your model from Pick a Brick (again, this will be expensive). Digital Designer is a good way to get started with virtual building, however the part selection is kinda limited. I do know of a way to unlock a lot more parts in Digital Designer so if you go that route let me know and I'll tell you how to unlock them.
