50 foot cables?

I would use Canare 4S11 speaker cables from Ram Electronics.  A 25 foot pair with termination will run you $139.00 for the pair.  These blow away my $1500.00 pair of speaker cables. 

@OP, 
You might get lucky searching Ebay for used cables.
25 feet is long, but I scored a set of AQ Rockefeller cables a while ago.
They were the real deal and AQ cut them down and had them reterminated when I moved to monoblocks.
Also, check Hifi Shark. Use the filters to narrow down your search.
A little patience, and some luck should allow you to find a pair of cables.
(Just keep in mind, there are counterfeits out their, so do your due diligence. -That is try to find sellers who can provide a receipt of purchase from a dealer, if possible).
Bob  

+1 Miller.
However, I would do everything possible to knock down the length of those speaker cables. Even 25 feet long is crazy and IMO probably will have some negative sonic effect.

@ tuberist,

"Even 25 feet long is crazy and IMO probably will have some negative sonic effect."
I have been using 25 foot long speaker cables since 1992.  Let me assure you their is no sound degradation.  I spoke with McIntosh the manufacturer of my amp and was told there will be no sonic degradation of sound.  I checked with my speaker manufacturer(s) and Sandy Gross from GE told me there is no sonic degradation of sound, lastly I have a EE degree and know there is no sonic degradation of sound. 

My speaker cables are 11 gauge and 25 feet is nothing for a 300wpc solid state amp.  I wonder where everyone gets their information from?  For fun, I moved my speakers next to my amp and tried 5 foot cables and trust me, there was no difference between them and the 25 foot pair of the same cable.   If there was, do you think I would have 50K of equipment in my listening room if the cables degraded the sound?