Keep an eye on the speaker listings here on Audiogon. Allison speakers do come up for sale once in a while. You could also place a wanted ad. But I've got to say that finding a pair in mint condition will be difficult, to say the least.
I am a fan of Allisons and check the EBAY listings every now and then but the Allisons many times show up in really abismal condition (somebody puts a radio shack woofer replacement in, for example, and then those delicate tweeter wire leads are broken, or tweeter is smashed all up etct)... the really 'minty' listings of Allisons are often overpriced. I think the model Four comes up more often and is a better speaker. Good luck. Would be a fun project for restoration/modding.