Are all fitness resistance bands the same?

[Aejo]

I recently started p90x and decided I would prefer to use resistance bands rather than the dumbbells and pull up bar. I was wondering if some brands of resistance bands were better than others, or are they pretty much all the same? Also, I do not know the "pounds" of resistance I should buy... should I go for the normal weight increments I typically use when working out at the gym? Also, where is a good place to buy them, cheap?

Thanks so much.