There are many outfits to buy from. I like "West Coast", they have a big variety. Shipping is significant, so you probably want to buy 3 or 4 soaps when you order, and those will probably last you a couple of years.
I have bought a lot of soap made by Stirling. The lather is good, and they give you 5.8 ounces of soap (most people give you 4.0 ounces), but they are often too aggressive with their scents and use of menthol. This of course is a matter of taste, but I enjoy a bit of balance in these things.
Ogallala bay rum is a wonderful soap. Good lather and smells wonderful.
Ghost Town Barber by chiseled face is a fantastic soap.
Ozark Mountain by Stirling, is great and my favorite of their soaps I have tried.
Orbit by Noble Otter is amazing, with just a touch of what I think is menthol that you don't notice right away.
Be aware that there are two dimensions to shave soap. One is the base, which produces the lather. The other is the scent. A great soap gets both right.
When I use up my current supply (which will take years), I'll try others on the Sharpologist list.
Barrbarr from Noble Otter
anything from Phoenix Artisan Accoutrements - given they are in Arizona and well regarded. Their website is a tangled mess, so try to order from some other seller.
Tom's Shaving Info / tom@mmto.org