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Keyboards 
As far as keyboards go, the top contenders are probably Logitech and Microsoft. If you want, you can buy both the keyboard and the mouse together in a combo/bundle. This sometimes saves money but mainly just makes things easier when buying.
The two main options when buying a keyboard is deciding between a regular or wireless keyboard. The advantages of having a wireless keyboard is more freedom and less wire clutter. The only disadvantage is having to replace the batteries all the time, but that can be solved by investing in a charger and rechargeable batteries.
Cost
Between $15 and $150(Average Cost: $20). There's a big difference in price because the most basic keyboard is very cheap, but more features such as wireless will cost more. It may be a good idea to get the keyboard and mouse together. Wireless combo sets will run about $50 - $150 .
Where to buy
TigerDirect: Input Devices