MyTradeWorld via Amazon offers the TFY Universal Car Headrest Tablet Mount
for $24.30. Coupon code "TABLET08" drops it to $14.58
. Plus, Amazon Prime members bag free shipping
. (Non-members can spend $49 to qualify for free shipping; otherwise, it adds $5.99.) That's the lowest total price we could find by $4, outside of the mention below. It supports most 6" to 11" tablets.
A close price: Billshop2009 via eBay offers it for $15.26 with free shipping