Just bought a ZX Interface 1 from someone that I connected with on the "Sinclair for Sale" Facebook group. He sent me pics of his stash and says he likes fixing things (i.e. it's not his main business but a side hobby). His prices aren't bad (though you can probably hunt for slightly cheaper) and he bundles stuff, which can reduce cost. His shipping is low (at least from Europe to US).
Here are some pics of his Sinclair stock (a few QL's on the bottom shelf):
Let me know if anyone is interested and I'll get you his email...if there is enough interest I can see if he's fine with me posting his email here.