Hoffman has a small one...Does anyone have a better source for .68 Caps? The smallest ones that I found were big. I think I bought them from Antique Electronics.
.68uf/250v 684
250 volt Panasonic Polyester film capacitor - cathode bypass cap
16.8mm wide x 5.39mm tall x 5.3mm thick
Price each = $1.00
I have some small M150s that I've used in 3 different amps. I don't know where I found them but they were out there back when M150s were yellow. Look on the right end of the board on this Plexi-6V6...
http://sluckeyamps.com/6v6plexi/P-6V6_05_big.jpg