I can only guess "A" refers to cylinder one? If so, it's good news as it's the easiest one to get too 🙂
Before you buy anything give the wiring a good look over. Check for any loose or bad connections, damage, or any weirdness in general.
If it all looks good, try swapping the coil packs between cylinder 1 and 3, clear the code and test again. If the fault moves to cylinder 3, it's most likely the coil pack. If not, it could be the wiring or potentially the spark plug?
Out of interest, how many miles has your car covered, have you ever had the spark plugs changed and have you had any work done recently?
From past experience cheaper aftermarket coils can be bit funky, so I'd recommend going OEM where possible and failing that, a manufacturer you trust or somewhere with a good warranty.
Looking the parts up... Toyota seem to have had 3 revisions, with "SU003-04713" being the latest.
Subaru may have done the same but I can only see one part number that active: "22433AA650"
I seem to recall members paying around £190 at dealerships but I may be wrong. It's worth calling you local Subaru and Toyota dealers to check.
As for online...
If you order from overseas looks like you can get a decent deal, even though Amazon!
If you look at the options, Amazon themselves sell this item for £136.08 with free 7-9 day postage from the USA. Or you can pay £8.80 for 2-5 day shipping.
Buy TOYOTA Parts-Coil Assy-IGN (SU003-04713) at Amazon UK. Free delivery on eligible orders.
www.amazon.co.uk
I also found this one
SU003-04713 GENUINE OEM IGNITION COIL ASSY SU00304713 | eBay, from the UAE.
As well as this one
Genuine OEM Subaru 22433AA652 Ignition Coil On Plug | eBay from the USA. Which isn't subject to any extra import charges, so the price you see is what you pay 👍
I did also find this
SUREFIRE for SUBARU BRZ 2.0 FA20 PERFORMANCE IGNITION COILPACK FK0438 SET x 4 | eBay, which looks to be an absolute steal! I'm still in two minds about it but they do have 100% positive feedback.