You can find a lot of this stuff at online retailers like Toys R Us, London Drugs, Walmart, etc.. on sale. Finding a good sale on a set that has high end chase cards in it, is the way to go value wise.
Otherwise online hobby shops are a good place to look. These are some of the cheaper ones I've found. I buy sports cards from them, never Pokemon, but these places all have a very solid reputation in the hobby world and are legit:
http://www.mirajtrading.com/
https://www.grizzlysportscards.com/
https://zephyrepic.com/
https://cloutsnchara.com/
If you have a way to ship from the USA, it's probably much cheaper:
https://www.dacardworld.com/
https://www.steelcitycollectibles.com/
https://www.blowoutcards.com/
I don't know anything about Pokemon cards though. So can't comment on what actual sets to buy.