Decide About Components
It is important for you to decide about components you want with your audio system. This will help you choose specifications of the material you need. This should include audio quality, sound system and additional hook-up.
Building Own Amplifier
If you have some training and are confident working with electronic components, you can build your own custom amplifiers. You can source components through different sources, even from online auction sites. There is a fair chance you can buy components in used quality from a shop or online. You better make a list of components and search online.
Selection of Brand
If you want to use quality brand products, you can search online for them. It is entirely up to you what level of quality of audio you want. If you want to use quality components, you can search online for different brands.
You can search and source for components locally. Sometimes, you can find quality products in your own area, either at local shops. Especially if you are building your own amplifier system, you can source all components at shops. You can discuss with local shopkeepers, and a handy person can source from the stores he buys stuff for himself.
Before buying components, you should consider compatibility of your existing components. If you buy new components, which are not compatible to the existing components, they will not fit in and even if they do, they might not work properly.
Consider Your Budget
Pricing is an important factor, so consider your budget beforehand. Buying quality components will also require additional investment and you should be able to spend the required amount. You might be able to get all components at discounted rates if you buy used components or buy online, without compromising the quality.