K and M Bricks on Bricklink appears to have a good deal (and plenty of supply) on the Transparent Neon Green Miru mask. I could not determine the cost of shipping without actually placing the order, but you can always send them a message asking for an estimate before you place the order.

The masks are otherwise listed for $8.00 USD. Even if the shipping turns out to cost as much as the mask itself, it still comes up cheaper than any listing for the same mask on eBay. Since they have plenty of stock (over 80 at time of this post), you could save even more on combined shipping if you happen to need more than one for a MOC, or have some local friends looking to grab the mask as well.