To answer your question, there are a number of ways you can earn apples, although you cannot find them in explore. The most common way to get them is Barter > Search > and scroll down to search for apples on sales. They sell for about 1400 mush usually. You can also advertise in sales chat/ forum want ads that you are looking to pay for apples. Ocassionally there may be apples in forum auctions, or offered as prizes for forum contests. Another way is during under games during the Full Moon Celebration, which lasts from the 5th to 9th of each month, you can win apples if you make it to the Lightbringers leaderboards. Under Pack, you can also go to free apples and mushrooms, and you will win apples if you are a top game referrer. And of course you can always buy apples with real money under Barter > Apples.

Thank you so much Life on Mars, that really helped me alot.

It's been a year since this question was answered so the prices for apples are a little different now. :)

Now apples go for around 1000-1250 mushrooms.