For my simple QRP Transmitter I needed a low pass filter. I found a nice filter design in the book QRP Basics. As I learned the component values have been calculated and published by Ed Wetherbold, W3NQN. Only standard values for capacitors are used:
For QRP power one can use Amidon toroid cores T37-2. For power of up to 200 Watts the T50-2 cores have to be used. The Amidon cores can be ordered from Kits and Parts, where you can also find an online calculator to determine the amount of windings for a given inductivity. Further information and component values for filters on other bands can be found on the technical pages of the G-QRP club.