I'd like to upgrade my speaker wire

You would be wise to consider Luna Cables.  They come in 5 grades.  I just purchased a complete set (speaker, IC, power cords).  Each time I replaced one of my Naim cables with a Luna cable, it drastically improved the sound.  The difference was truly shocking.  The higher grades (Blue, Ted, and Black) are pricey but the entry level (Grey) or the next one up (Orange) are quite reasonable.  They are hand made in Canada.

I agree with @Whoopycat, the Duelund 16GA wire is a great step-up from just about any standard multi-strand wires on the market. You can buy the wire and twist it yourself for the runs you need.  If you need more power then the Duelund 12GA is also very good but more costly. 

Luna Cables also use tin-plated wire (maybe they even use the Duelund inside), but they produce finished speaker cables, and don't just supply the wire - which is what I interpreted the OP's request as.

I use 18-8 thermostat wire, $85 for 250 feet, you'd need 160 feet.  Solid copper, 8x18 insulated wires per "conductor" = 9 gauge, use the whole cable for each terminal.  It's easy to strip and terminate if you need to terminate it.  If your connections on the speaker and amp can accommodate the size of the twisted together wire so much the better.  If you need to terminate you can get gold plated copper spade lugs on eBay pretty cheaply.  The cables are a bit unwieldy, but they're tamable.  I use 3/8 inch split mesh around the pairs with heat shrink at the ends for a nicer appearance. 

I can hear the naysayers already, but if you haven't tried it...  Some friends of mine have and have made the switch.

long interconnects, short speaker cables.