MX5 Parts have an online store with an extensive set of parts for the MX-5 / Eunos.
We haven't used MX-5 Restorer yet but, they have a good reputation for things like rear sill repairs. There are some good videos on their website that are worth watching too.
I discovered Suffolk Mazda via their eBay store. They are within a few miles of me, which is very handy and have loads of second-hand parts at very competitive prices.
So far I've used them for odd bolts and fixings, a set of alloy wheels and my hard top.