KYB makes replacements, or else Subaru OEM parts.
Personally would replace in pairs.
Of the top of my head I paid around €30 a piece from Castle in Bray when I did the ones on the Blob a few years back (not 100% sure though as has been a while). The KYB parts were about 40% cheaper, but was going to take a few weeks to get them and dealer was 2 days delivery.