I need a new couple of stem bolts, what size is this bolt?

https://www.dropbox.com/sc/ahnp7utblf06jyw/AAAw-lCx4F9c6Y0MmXiyNRRva

Is it m4 * 15mm? what do you call the top – square head?
where can I get some? I assume I want stainless steel? titanium any stronger?

Socket cap, eBay. Steel stronger than ti. A4 stainless if you don’t like rust.

M5x15mm cap screw
Plenty on ebay

You could even go aluminium if you wanted as its not load bearing.

http://m.ebay.co.uk/itm/360835494845?nav=SEARCH&varId=630221129263

As onza says, if there low torque then anodized alloy bolts will be fine
http://m.ebay.co.uk/itm/350977667400?nav=SEARCH&varId=620228774431

You could even go aluminium if you wanted as its not load bearing.

This looks like an M4x15.

The dropbox page refers to it as a stem bolt.

DO NOT use Alu bolts on your stem!

I can only think that people are assuming it is the headset bolt to recommend alu and describe it as non-load bearing

Agree with Dan. Alu stem bolts are a bad idea IMO. Speaking as someone who uses Alu bolts.

Sorry, yes I thought we were talking about the seem cap bolt for preloading the headset. The above comments are correct if you mean a stem clamping bolt for pinching the stem down onto the steerer. That is load bearing and is best served with a steel bolt.

Reading it again, you mention a couple of bolts so my earlier helpful post was complete bollocks.

An interesting page on fastener strengths here. Replace fasteners with at least a matching strength

Why?

If your bike comes with a steel headset top cap bolt, or mech cable pinch bolts then you can use alu bolts just fine.