As with any consumable item, the best thing to do can be to put the part number into Google Shopping (which gets rid of a lot of the 'noise') and try one or two of your favourite suppliers directly. In this case, the part number is S041332 according to the Epson web site
, which leads to these Google results
However, some supplies list VAT inclusive prices with Google, some list VAT exclusive and some list prices that are just wrong. Shipping isn't taken into account either. The cheapest supplier currently on Google (Memory Megastore) seems to have some very odd policies. I find that LambdaTek tend to be about the cheapest for many computer products - right now, they want Â£10.89 a pack
(including VAT but not shipping), though their prices fluctuate and their shipping policies may favour a larger order.
If you have a decent rotary trimmer (or similar) you could try cutting down one A3 sheet into two A4 sheets. I have a Rotatrim and would be prepared to give it a go - though there's always a fear of damaging the surface of photo paper.