You can get reels like these

[Linked Image from waterfordhire.com]

Various connector setups like :
http://www.olson.co.uk/iec309.htm

Most of these would have "3 Core 2.5mm² Arctic Grade" cable

[Linked Image from justgenerators.co.uk]

You might be best to ask for local advice in Belgium though as wiring regulations across europe while fairly harmonised are not absolutely identical.

Regs on industrial setups in particular can be fairly tight and US equipment can be way off standard.

I couldn't stress how important it would be to ensure that your customer gives you advice.

Can you supply the gear to the customer without connectors and with connection instructions?