Richard,
I'm not sure who carries these things in a full line these days - if anyone. Trend did have them until a few years ago but appear now to have gone over fully to carbide. I think cheap carbide imports killed HSS in this usage.
Bosch do/did a few, as did/do Leigh (for their jig) you could also try asking Wealden. Depending what sizes you want, end mills and slot cutters are available in half, five sixteenth, three eighths and quarter inch shank sizes from places like Tilgear. These common sizes will usually fit a router collet or suitable adapter.
You won't find HSS router cutters in any but straight bits nowadays - or the dovetails available from Woodrat. HSS dovetail bits for milling machines will have the wrong angle/size for wood.
Their superior sharpness notwithstanding, if I were going into production mode, I would opt for carbide cutters as they will last far longer between sharpenings. I also favour the replaceable insert type of straight cutter (I use KWO but I see Trend do them now)and I find these both sharp and long lasting. Despite being - or rather because they are a single cutter, they clear the waste very quickly, avoiding damaging heat build up and they also make a super job of cutting clean mortices (better in my view than the spiral solid carbide type).