It seems there has been a shortage of Samsung Finesse handsets for Straight Talk -they are no longer available on Walmart’s website and most Walmart stores, and whether you can buy one from Straight Talk directly seems to depend on the day that you try.

I’ve gotten a lot of questions lately from people wondering where they can find this phone. One way that I’ve found to reliable track down this model is by shopping on eBay. It seems there are several new auctions available daily, so you might want to check out that site by searching here.
Search for the Samsung Finesse on eBay

As always with eBay, you’ll need to make sure you buy from a reputable seller, but in general it seems that transferring even a used Straight Talk phone is pretty painless.