With the recent release of HENkaku, followed by an update from Sony that patched the hack, people are looking for ways to buy a Vita running firmware 3.60 (or lower) at reasonable prices.
In that kind of situation, and given how recent the hack is, your best bet is to buy a new PS Vita. At the time of this writing, it is pretty much guaranteed that any new PS Vita will ship with a firmware 3.60 or lower.
Conversely, you’ll want to avoid “refurbished” and “used” consoles, unless the seller can guarantee the console ships with a lower firmware (and sellers who can do that will most likely charge you extra for that kind of guarantee). The reason is simple: most shops that sell used consoles go through a process of resetting the device to factory settings and updating it before selling it. As such, you’re pretty much guaranteed to get firmware 3.61 or higher if you buy a used PS Vita from a store or online.
So where to get a 3.60 vita? The obvious choice is going to be to buy it new from Amazon. With their return policy, you’re also safe if the product doesn’t match your expectations. People who want to be extra paranoid can get the Borderlands 2 bundle, which was released in 2014. A new version of this one will ship with a lower firmware.
Similarly, if you buy a PS TV new, you’re guaranteed to have a firmware below 3.60, as the production of PS TV was discontinued a while ago. A such, new PS TVs are guaranteed to be on a low firmware.