I've got a v5 WRX (1999) and have the 2-pot rear calipers and conversion brackets. What discs do I need to get?

 

2003 wrx rear disks

 

^^^ Correct.
To expand... Your handbrake drum diameter will be 170mm, new age WRX (2001-2007) is the same.
But cars with an R180 diff will have 190mm handbrakes, that STI / RA / Type R / JDM imports etc.

You can either buy OEM discs or upgrades from aftermartket people like godspeed or DBA
<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}-->https://www.godspeedbrakes.co.uk/dis...handbrake.html

https://dbabrakes.co.uk/catalogue/SU...82001-2005%29/

 

PS, have you already upgraded your front brakes? If not you'll have moved your brake balance rearward quite a bit and might find your rear wheels lock up first under hard braking. Which is not great for staying in a straight line!