That Tractor Supply price is not out of line and you touched on a big advantage- they are open on weekends, seems weekends when the gas outfits are closed are the most likely times to run out
They also will exchange any bottle as long as you own it and it is in date. The last Acetylene bottle I took in to them had been expired for about a year and they took it anyway telling me they would get it tested but they just did me a favor on that on. Get the biggest bottle you can handle because the fill price difference between the small bottles and the big ones is not very much and the price per cubic foot of gas is a heck of a lot cheaper when you fill a big tank. Sometimes it may be tempting to just forget the gas and use flux core but when you consider the cost difference between flux core and the solid wire the gas does not seem so expensive.