If you can pool your purchases with other players and order several pairs at a time, it often is cheaper to get them from Elderly. Their string prices are lower and their shipping flat rate. For example, for my uke class, am combining orders. In lots of 5, or more, I can get Aquila soprano nylguts for $4.50 with free shipping. If you can beat that, good.
HMS, Strings By Mail, JustStrings.com - the latter is especially good for oddball single guitar strings; for a while that was the only place I could find the Thomastik-Infeld chromium flatwound classical guitar strings that I use on my baritone.
I avoid Elderly unless I'm buying quite a few packages or something else with the strings. Their prices are not bad but their shipping works out pretty high on small purchases.
I'll chime in for Elderly as well. I use Worth BF's and love them. I have done business with Elderly for years. Fast delivery, easy transaction. Bought my strings in volume last time, won't need to order again anytime soon! Would love to visit Elderly in person someday.
Elderly is great if they have what you want. Strings by mail has more custom options, especially for Savarez, which don't seem to be available in custom diameters anywhere else - and I haven't experienced their shipping prices as outrageous? So, the best place has to be first, the place that has what you want/need