Hoosier has a good deal on the kGo, but like said, they do sell out pretty fast. If their sold out before you get a chance to get one, I would also check out DarkCity Vapor. They also sell the kGo and have a good deal on it.
Another option would be an E-Power, or like already mentioned, a Riva. They are both 3.7V PVs like the kGo and have 510 connections.
When I first started, it was between the kGo and E-Power. Their both great PVs, but I went with the E-Power for a few reasons. The E-Power uses non-proprietary
batteries, which are removable, and thats the biggest difference of the E-Power compared to proprietary battery PVs like the kGo/Riva/eGo. E-Power
batteries cost $4-6 compared to kGo/Riva/eGo batteries that cost $14-20, and they last a lot longer before they need to be replaced. Also everything is replaceable on the E-Power, which is the other main different compared to the proprietary battery PVs. With the kGo/eGo/Riva, everything is in one unit, so if a battery needs to be replaced or a switch fails, you have to choice but to replace the whole unit everytime. With the E-Power, everything is seperate and can be bought individually. So if you need to replace a battery, you can but just a $5 battery. If a switch fails, you can buy just a $10 switch, and continue to use your existing tube and battery. That is one of the things I really like about the E-power bc the replaceable parts make it cheaper to maintain. I also like that its much easier to carry a spare battery for the E-Power. The batteries are about the size of a AA, but alittle longer, so its really easy to carry a spare battery in your pocket or bag, and if needed, unscrew the switch and pop in a fresh battery, so you really never have to worry about being stuck with a dead battery. The E-power batteries are 1050mah and I get on avg a day to a day and a half on one battery. With the proprietary battery PVs, the only way to carry a spare battery is to carry a second PV. Its not a huge deal, but its def something to consider, and personally I find it much easier to carry one PV with a small spare battery, then have to carry 2 PVs.
The kGo is def a really nice PV though, even though it uses proprietary batteries. The batteries are 1100mah so they last a long time and it gives a great vape.
Overall I think youd be really happy with any of the 3.7V PVs, and if you cant find a kGo anywhere, the E-Power or Riva would def be a second option.
Goodluck!