I am away at the moment so don't have the details to hand, but there is an excellent company at the farm off the Watton Road near the Dane End turnoff that does wrought iron work and fitting, who I would heartily recommend. Will look them up after Wednesday if of use to you.
Try the Jones Baker Engineering Company on 020 8808 8196. Mick Jones 07950 440980 lives in Ware or his son Steve on 07930662020. They would be able to sort out suitable posts and give you a price for installation.
I think there is a place up at Gentlemens Field (just across the A10 roundabout from Watton Road) that do iron gates etc... They may be able to help you or point you in the direction of someone who can.