Electrolytic capacitors in guitar amps in odd configuration?
These aren't electrolytic capacitors. They are film capacitors.

The electrolytic capacitor symbol has likely been abused to note which side of the capacitor to connects to the outer foil of the capacitor. 

Back in the days capacitors have been built by having two metal foils separated with insulating film. If you roll them one metal film will be on the outside. This is usally marked on the part somehow because having the outer side connected to the lower impedance point in the circuit gives a slight advantage for noise immunity.

For buying parts, the classic brands for guitar amplifier capacitors still are: 


Malloy 150 for british sounding amplifiers (Marshall).
Sprangue Orange Drops for a US sound (Mesa Boogie).


If you google for them you'll find online shops selling these.