There are a large number of retailers worldwide who sell ukuleles online. I have only included a couple of local (to me) retailers as well as Kala who has a wide range of ukulele related items but not the instruments themselves.
That's because it is ALWAYS a good idea to play a uke before you buy one and trying before you buy if purchasing second-hand or going to your local music store is recommended.
Buying online is a safe(ish) bet if you know/trust the brand and or have owned or played a particular uke before.
Kala sells accessories from their site but not ukes.
Jack's Music have Cole Clark, Leho, a bamboo model by Veelah and their own brand, Jack's, for sale online.