The usi mics do appear with regularity. The key to buying them is to get on the mailing list. They will let you know the exact date and time they will be up for sale.
I don’t have experience with Naiant—they look interesting. For the price of a pair of usis, you can get a used Røde NT4 or a pair of NT5s. For stereo recording convenience, the NT4s are very easy to use and sound really great. If you are recording outdoors, because they are super sensitive, you will absolutely need to buy a windscreen or a blimp.