here are links to some of the usual suspects for combat worthy shield bosses...

https://www.celticwebmerchant.com/en...shield-bosses/

https://jelldragon.com/collections/shield-accessories/

https://www.wulflund.com/armour/shie...ses/?vp-page=1

There are a few others, but for the most part they overlap styles with the above and the ones listed have a known track record for actually delivering product without hassle. You wouldn't believe how often armour merchants disappear in the night with folk's money.
There's also a fairly significant underground market for this kinda one-off stuff. If you'd like i can pass on the link to an armoured combat/armour construction forum populated by cats that build and fight in this stuff all the time and could make up a custom piece.