"Overclocking" cannot be purchased or obtained, as it is not a material possesion. The term overclocking refers to a user increases the clock speed of either memory, processor, or video beyond the speed it is sold to run at.
For example, if you an AMD Athlon 3200+ which runs at a stock speed of 2.0Ghz, the multiplier will be set at 10 and the front side bus (or in this case, the hypertransport) will be set to 200.
200 x 10 = 2000
You may access the FSB/HTT number via the BIOS in order to increase or decrease it. For example, say I increased the HTT by 10mhz, the equation would then look like the following.
210 x 10 = 2100
You have now successfully increased your CPU clock speed by 100mhz.
Be warned, overclocking is dangerous. It increases core temperature, increases the risk of instability, and it voids your warranty. It should not be attempted if you do not have a firm grasp of the concept and are not prepared to troubleshoot issues that may arise.
The BIOS (basic input output system) is a piece of software which is built into your motherboard, and controls the basic configuration of your computer. Do not tamper with any setting in the BIOS is you are not familar with it.
The BIOS will have come with your motherboard though, it's the foundation for your PC, your computer would not boot without a proper, working BIOS. You will not have to buy it.