Which octagonal gate model type to replace Hori EX2 square gate?

looking to replace my square gate on my Hori EX2 fightstick with an octagonal gate, and need to know which model/type will be compatible?

note: i have not modded the joystick.

Also any online shops you can recommend for me to purchase the item from the UK?

The Sanwa GT-Y is what you’re looking for. You will need to drill some small holes in the gate itself so that it can be screwed to the base though (it’s really easy though, guide to doing it is here: http://www.neo-geo.com/forums/showthread.php?149488-Easy-DOA-Stick-Mod-For-Those-of-You-Wanting-To-Play-Some-SF2-on-XBLA&referrerid=13029 .

As for UK shops, Gremlin Solutions would be a good place for you: http://gremlinsolutions.co.uk/ordersanwajoystickparts.php

what about the Seimitsu LS-56 octo gate? will that work?

Doubtful, I own one myself and the holes don’t line up the way a GT-Y does, and it’s smaller.