NPO relates to the temperature dependent behaviour of the capacitor - NPO / COG capacitors have lower change in capacitance as a result of change in temperature - as little as 0.01% over typical room temperatures with change of 25 degrees C, and 0.1% over the entire operating temperature which is well beyond survivable for the thereminist!

X7R capacitors vary about 5% over 25 degrees C, Y5(x) can vary by 20% over 25 degrees C.

This is the VARIATION in capacitance due to temperature, not the tolerance - Tolerance spacifies the possible variation one can get at a fixed temperature, between parts - this can easily be +/- 10%, but is not important (from a drift perspective) - its only important if one has untunable elements and need to have close tolerance.

You do need to buy NPO / COG capacitors for any tuning circuit - Or you certainly need to know what the dielectric is.. Buying a "random" ceramic capacitor is never a good idea.

You can use other capacitors if you know what you are doing - one can deliberately use a capacitor with a temperature coefficient which counters some other coefficient (like that of your inductor) so that one gets really stable operation..

But IMO, you are best sticking with NPO / COG.. Every good supplier will be able to provide these - any supplier who doesnt know what you are talking about aint worth using..

Digikey NPO COG Selected capacitors: Digikey US - NPO / COG - Select values.

Farnell / Element 14, Newark, and probably every other big supplier has a huge range of parts - just enter COG and/or NPO in the search, select Ceramic capacitors, select Through-hole.. Then pick the voltage you need (50 Volts for oscillator circuits, 200V for antenna circuits).

Fred.