Hi,
Ive had the same idea, looking to change my factory seatbelts to Yellow, to match my ordered MGP caliper covers and fitting a yellow stitch Alcantara steering wheel also.
I found a local supplier to me in Notts, UK on ebay, here is the pricing for reference (supply & fit):


Here is the ebay listing: https://www.ebay.co.uk/itm/224135149...&segname=11051