I've found the best prices (including shipping) on eBay, but selection can be limited. Uke Republic's shipping is reasonable and the selection is good. And now that I've fallen for Martin M600s, I buy those on Amazon when I'm a few dollars short of the $25 needed to get free shipping if I'm ordering something else.
Wow,....you like the Martin M600 set? I had a bad A string on my Moore Bettah soprano and needed a replacement quickly.
I tried them recently because it was the only soprano uke set i could find "locally". They sounded pretty good, but the slinky
and thin feel of the A string especially was a turn off to me.
Once i got a few sets of Aquila nylgut stings in i replaced them immededialy, and am much happier with the feel.
The thicker strings are also a bit easier on me already sore fingers. (i've been putting a lot of time in!)
BTW, i picked up multiple sets of both soprano and tenor nylgut strings (3 sets each at a slightly discounted price) at
Elderly Instruments. I was buying a tenor "Uke Crazy" case anyway and paying the shipping on that, so the strings
were an add on that didn't cost me anything extra to have shipped.
The basic Aquila string costs at Elderly are cheaper than a string speciality store like "Just Strings", btw, so i went there.