Hi Markus

I'm looking for reasonably priced QSFP (40GE) to SFP+ (10GE) twinax breakout cables (3m long or so). I can order via ebay for ~$150 but I was wondering whether there's a local distributor who has a stock for them and doesn't sell them at 500CHF or more... Any recommendation?

Sadly, nothing in our stock currently.

Also, what kind of adapter would I need to break out a QSFP to 4 SFP+ sockets (where I could then attach fiber SFP+)? Haven't seem something like that, but perhaps I'm searching wrong?

You’re search is correct. Technically it will be a challenge to build such a part as you need to provided power and management-signaling for 4 x SFP+ (in total 6 Watt power consumption combined to one single QSFP+ port). Regular QSFP+ to SFP+ Breakout DACs are „just“ and extension of the high speed lanes via the Twinax-cable.

Can you explain your application in more detail? An alternative for such a „port-splitter“ (QSFP+ port to 4 x SFP+ port) could be that you use QSFP+ SR4 (4 x 850nm) or QSFP+ Parallel LR4 (4 x 1310nm - https://www.flexoptix.net/en/qsfp-plus-parallel-lr4-transceiver-40-gigabit- sm-1310nm-10km-7db-ddm-dom.html) straight ahead. Combine it with a MPO breakout cable (Multimode or Singlemode) and you can connect 4 x SFP+ as counterpart interface directly. The majority of QSFP+ based gear supports 4 x 10G operations of the QSFP+ port today.

Cheers

Thomas