When it comes to cards for teaching, two companies come to mind: Trend and School Zone. Both of these produce excellent cards for their cost and will last you years of useful service.
Trend is the leader of the card war because they have a huge volume of cards to chose from. Their boxed sets of sight words, word families and verb cards are sets every English teacher should have for teaching ESL. The boxes protect and keep the cards from getting lost/damaged while the double sided format makes them flexible for reading practice.
I would say though, stay away from the set vocabulary-like opposites- as these are not a good cost to value product. The card sets can be used easily, but I have found any matching set type product to be hard to use in lessons and the number of cards to cost is dramatically skewed.
School Zone relies on drawings instead of photographs, but because they offer more game based cards they are a very valid company to check out. Their sets include some important vocabulary, especially adjectives, that aren’t found on Trend cards. Games like Go Fish Alphabet are a must for small classes, not only for the letter reading, but the review of animal names that go along with the game.
Cards, like any item in a classroom, are a tool for teachers. How you use them is where the students learn. Having the best cards you can gives you the best chance to impact and improve their ability to use the target language. I highly recommend both of these companies for the products they produce in cards.
Keep writing, and teaching-that’s good too!