SME 309 HTA Key. Does anybody know how to get one or an alternative?

Appears to be a Torx head.  If so, order a set from Amazon.  
https://www.amazon.com/Syntus-Precision-Screwdriver-Electronics-Cellphone/dp/B071PB4RPV/ref=sxin_23_...

It's a 3mm Allen socket.

danmar123
Are you sure we are talking about the same thing HTA Key.? It’s not an Allen wrench.
Looks more like a torx head.

No, my apologies. Look @ page 7- https://sme.co.uk/audio/wp-content/uploads/sites/1/2016/12/SME-series-300-instructions.pdf

I just checked my box, & it's a torque wrench. 

HTA key is similar to a T6 torx.

A Torx head is not the same as a hex head.  A hex screw has a six sided depression, while the Torx depression has a 6-sided star shape.

From the instruction manual danmar posted, you are looking for a common allen hex driver.  Get a metric set with enough small sizes and you should find one that fits perfectly.  If you want to get fancy, you can get a ball-ended hex wrench set.  It is this ball-ended type that SME apparently supplies with their tonearms.  My Basis Vector arm came with ball-ended hex wrenches too.  They essentially work the same as regular hex wrenches, but they are easier to insert because you can come in at an angle and they will still fit. 

Sorry, but the SME HTA key is still a T6 torx.

refer to page 14 (Step 421):
https://sme.co.uk/audio/wp-content/uploads/sites/1/2016/12/SME-series-iv-instructions.pdf

 Analogue Seduction sells the SME HTA key.  I just saw the picture and it does look like a Torx head.  The item shown is pretty expensive, so a set of Torx head screwdrivers is a good alternative.

I have an SME V and my HTA key is definitely a Torx-style wrench. It's not an Allen key.