Well, i'm not an expert in this area, i've done more reading on lizards, however, if you want to get a sexed animal, then it will cost more, especially if you go for female. Also, it would depend upon their previous owner as to how tame they are.
Some breeds can be tame, like milk snakes and corn snakes, which are relatively common first timers, and also are good feeders (some snakes can be prone to problems feeding, which you don't want first time). Sizes of milksnakes can range to extremely tiny (kingsnake) to about 6ft, and most commonly available ones, at least in UK, are good to handle. Cornsnakes are generally good too.
Some pythons (like certain kinds of carpet pythons) can be very good pets, a little trickier to care for, but still good pets. Also, i think can be more trickier to tame down.
With some species, there is a greater chance that they will be aggressive and can have larger personality variability in the species.
You will normally need a source of UVB and UVA (unless nocturnal), and some sort of heat source with thermostat, which it doesn't have direct contact with, as otherwise will burn itself.
The best way to find out about them is to look round on the internet. Search on different advertisement/classified sites as well as reputable shops/breeders (google is a God-send on this one!), see what's available, read up on their care/tempremants. Please, remember, to read up on them properly!!! There is care sheets on most common snakes, so there is NO excuse.
Just look around.
And, the biggest snake is an anaconda, which, i do believe you can buy as a pet (i think i've seen it advertised in passing, but def would not recommend, they are big enough to eat a small child). Remember the bigger the snake, the more expensive. Make sure you can afford its upkeep before you buy it. And, make sure you don't mind feeding it frozen mice/rats, which will be required with large snakes.
And, if you get a snake that isn't full grown, make sure you don't go and buy the viv size it will need when its older (tank = viv = vivarium), too much space makes the snake feel insecure, and can make it not feed!
Also think about vet bills, and consider insurance. Generally reptiles are very hardy if cared for properly, but make sure you have the buffer there.