To find cheap Pokemon cards for sale as of 2015, browse websites that sell Pokemon cards, such as TrollAndToad.com and PokeOrder.com. Other options include Amazon.com and eBay.com. Theme decks, collections and tins are another option for acquiring multiple cards at an inexpensive price.
When buying individual cards, prices vary based on the card and the shop. Check multiple shops to compare prices before purchasing anything. If there's a local card store in your area, check prices there as well.
Collections and tins typically provide some pre-determined cards and booster packs, which contain random cards. If you want the pre-determined cards, these are an inexpensive way to acquire them, with the possibility of getting other useful cards. If you're looking for a specific card, it's more cost-effective to purchase that card alone.
The quality of theme decks varies, but they list their contents, so you know exactly what cards you're getting. Theme decks are a good source of energy cards, as they usually have about 20. The only alternate method of acquiring a large amount of energy cards is buying them in bulk online.
The most cost-effective method of purchasing Pokemon cards varies depending on your goals. If you're collecting cards, compare prices for the cards you want online. If you're constructing a deck for competitive play, determine the deck you want and the necessary cards. Use theme decks, collections and tins for cards you need multiple copies of, and complete the deck by purchasing single cards when necessary.