Hey Pokewho !
It all depends on what particular cards that you are looking for. It will depend whether you are looking for cards to collect or build a tournament worthy deck. I would say eBay is a good place to get single cards as tournament worthy cards can be bid on and perhaps gained at lower prices. For example I got a Landorus-EX for £10 when they normally sell for £16-22 each !
Some EXs such as Mewtwo-EX,Darkrai-EX and Rayquaza-EX have been reprinted in tins so they will be cheaper. Soon Keldeo-EX will be printed in a tin along with Black and White Kyurem-EX.
For packs you could look around and see what is your best local supplier. Most people will use Troll and Toad for TCG stuff in the USA or international buyers who want their US items shipped to them.
Some commons are better/easier acquired through buying booster boxes, but that depends if you are looking for EXs from that particular set. For example supporters such as Professor Juniper and N are in Dark Explorers but if you have no interest in the EXs in that set, then you may be better off just buying them in singles.
Plasma Storm is a good set to purchase if you're looking to get started in the game, same with Boundaries crossed.