Yep, it's "We Don't Rent Pigs". You can buy it on eBay!!
Lonesome Dove Sign Hat Creek Cattle Company Copper Hand - eBay (item 370092424549 end time Jun-28-09 19:06:43 PDT)
Here's a quote from the movie from Gus about the sign...
"Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop."
More quotes from the series