For sure the best deal for the money by far are the Rondo bass ukes,
http://www.rondomusic.com (in the search, type bass uke).
What are flat strings?
There are basically two versions of bass uke strings, fat poly and metal round wound nylon core. On the poly side Aquila makes the Thundergut off white used by many bass uke manufacturers, and they also make a bit thinner version called Thunder Reds.
Kala uses Road Toad black Pahoehoe (pronounced pah-ho-eh-ho-eh) which also come in bright red, bright yellow and bright green directly from Road Toad. They also use Rumblers, but I don't know anything about them.
The metal wound are made by Pyramid, but recently Kala put out a version that are a bit thicker and less costly than Pyramids. I have a couple of sets of Kalas and like them, but they're noisy because of the round wound metal.
The problem with Aquila Thunders is they're sticky, which also causes more finger noise. They also sausage, they get dents where they hit the frets, but Aquila just sent me a set of Thunderguts with a new formula that's supposed to fix those problems. I'm going to test them tonight after my gig today.
I like the Pahoehoe, they'e very smooth, but they take longer to settle into tune than the Thunderguts.