A BCI boa is a good step up from ball pythons. Husbandry wise, they are actually easier (more forgiving) but they do of course grow much larger, hence not considered a 'beginner snake'.

I'd suggest getting a baby from a reputable breeder. Make sure you have room for a 6' long cage when it's an adult, and when it's full grown, you'll want a second person with you to help you handle it, just in case.