Yup, a 10g would be fine for at least his first year, probably 1.5-2 yrs depending on how fast he grew. And yes, rinse it out with a diluted bleach solution, I've also used vinegar on old fish tanks, as it's an acid and can dissolve away tough gunk left over by fish; just make sure to rinse, rinse, and then rinse some more.

Just make sure that when you put the snake in it that the lid is ultra secure, corn snakes (especially hatchlings) are notorious escape artists. As for finding a corn, if you have a trust worthy pet store then by all means go through them. But shows are also a great source as well.