At Walmart, over here it's really cheap like I bought this and a few eyeshadows for $1 each and a nice full big brush for $2. The brush was good quality and the eyeshadow showed up very well, I didn't like the liquid eyeliner but the pencil worked great.
The cheapest I found elf was at target. They have a lot of their product their and their prices are the same or almost the same it is on online. It's also cheaper because you dont have to pay for shipping. You can see the product before buying and you dont have to wait to try It!
You can find it the cheapest directly on their website at www.elfcosmetics.com. Here you will find all of their products (a wide variety!!!)
Not only do you get FREE SHIPPING on orders over $25, but there is also a code for a FREE GIFT on every order.
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Honestly cheapest place would be ELF's official website itself. They almost always have sales / free shipping and you can pick a variety of their stuff at once that other retailer may not carry. Shipping can take some time but its worth it.
In the elf website, and if you sign up for their emails they send you notifications to when they are having sales and sometimes they will have coupon codes for free shipping and discounts on kits they put together.