There are a lot of vendors who sell 510s. Among those in the suppliers forum are: Cignot, Cigeasy, MadVapes, and Nhaler. These are all good, reliable suppliers as far as I know. They are not the only vendors for the 510, as it is one of the most popular ecigs. Be aware that the one drawback of the 510 is it's short battery life on the standard model. For this reason, most users of the 510 std model like to get a pcc (personal charging case) to go with it for charging on the go. There is also a 510 xl (or mega) battery that has nearly double the battery life, but at the cost of being a little longer in size. And many, many users like the eGo, Riva, Hello 016 or similar larger battery model. While not huge by any means, these are a little bigger in diameter. But they have four times the battery life between recharges. VaporJunction sells the Riva at a very reasonable price for the kit. The eGo tends to be a little more expensive, but is also an excellent model. The Hello 016 has the longest battery life of any similar model, but is also slightly bigger than any similar model.
You will want to experiment a little. Many like cartomizers for their convenience. Others prefer cart/atty combination. Others like dripping. Each is a bit different and has it's own strengths and weaknesses.
KR808s are good too, are primarily a cartomizer model, but it is possible to use a 901 atomizer on a KR808D-1 battery.
Welcome Catie.
Enjoy the journey.