I buy them all on Aliexpress, so maybe you can too.
And no you don't have to buy the box to get a rare card but if you do the chance is higher and it's usually cheaper to buy a box with 20 packs than to buy 20 single packs. Individual packs are just packs from boxes anyway. Also, you can see the probabilities on the back of both the box and the pack.
Like this one for example, it means that 13 packs of the box will contain 3R+1SR+1PTR+1SSR or UR cards in each pack and the remaining 5 will contain 3R+1SR+1PTR+1LSR, SGR or SC