Hi Jonny,
Stainless would be ideal.
Depends whether you want 'em 'drilled' or 'undrilled' - with or without the various mounting holes
Undrilled
Afaik, the basic front and rear fenders are essentially the same on all '73-on disc-brake bikes, and some T160's (so '75) were fitted with stainless fenders. The only difference is whether the front edge of the front one and the rear edge of the rear one are 'rolled' or not (and obviously the T140D front is cut down from the more common one).
If you think you want undrilled and you want to keep the front and centre 'hoop' mountings, be aware that its a time-consuming 'mare of strapping, marking, drilling, strapping, marking, drilling for almost every hole; rush it (and sometimes even if you take your time
) and either the fender will be off-centre or you'll have big holes to allow 'adjustment' around the bolts.
Risking telling you something you know already, the 'hoop' mountings are crap at keeping disc-brake legs doing the same thing; later-'76-on "Bridge piece" mounting still isn't anything to write home about but it's an improvement. :bluduh So +1 to Wol's advice to change to the later front fender mounting (no changes necessary to sliders) ... and only five holes to drill. :thumb
Btw, when drilling, if you drill to size, ime sheet metal gives a trochoidal - three curved sided - hole rather than an actual circular one.
I drill slightly smaller and finish to size with a rotary tapered file in an electric drill.
Drilled
Might be harder to source a particular hole pattern at any given time, as - say - a '73-to-early-'76 'hoop'-mounted fender is different from a "Bridge piece"-mounted one. In addition to Jim's suggested sources, as the exchange rate is favourable, you might want to check
Autocycle Engineering in GB (who might supply Jim's suggestions but also retail and have a Fleabay shop. :thumb).
Btw, 'fraid here Wol isn't quite correct saying late production stainless fenders "should fit straight on"; '79-on, Meriden shortened the grab rail, so its fender bolt holes are closer to the rear frame mounting than the corresponding holes in a '73-'78 rear fender.
Hth.
Regards,