Hmmm just had a look on c24, cheapest 356 i saw was rm6489 no box papers, good condition. I suppose the price can't be helped la i reckon, it being a chronograph. Had a look at the TWC watch company (they're a japanese watch seller, available on c24), cheapest 356 brand new on leather is going at JPY238,300
https://www.thewatchcompany.com/sinn-instrument-chronographs-356-022-leather-cwae-blk-csw.htmlI reckon ur gonna have to order from outside la looks like, if Sinn won't send here n THG won't carry it. I faced the same issue when I enquired THG about the 556i that I wanted previously. I had ordered mine from a place called Timeless in Luxembourg, but for the life of me.I can't seem to find their site nor their c24 listings anymore. Between Timeless and TWC, these 2 had decent prices on brand new german timepieces.
One good thing though, u r probably right in thinking that 356 doesn't have much reps. That particular industry is based on demand volume n the price of the gen timepiece itself. There's little enough demand for Sinn reps as it is, let alone the 356 specifically. Checking out the usual suspects, I come across a bunch of other models but not 356. YMMV, but I reckon its a safe bet.