I recently went to Andalucia, and just before I left I recalled there being a shop in Seville that did big shoes. So I had a quick look through my list to the left and there indeed was Calzados Guellas. It's only a short walk over the river and down into the Los Remedios barrio, which is mercifully free of tourists and has a few nice cafes as well as all sorts of other shops down the main street. The shop is small but there is a lot of choice and the lady who runs it is very helpful. She's also happy to deliver to the UK and said it costs approx €35 (my Spanish is poor!). She seemed to go up to at least a 45.
Also in Andalusia, this time in Malaga and only a short distance from the main bus and train stations, is Zapatos de talla. This is part of a much bigger group called Gody, and they have a really good selection of shoes in larger sizes in their shop on C/ Cuarteles. The shop is open Mon-Sat 10am to 9pm and the assistants were very helpful despite my limited Spanish. I got a pair of the Hush Puppies sandals shown on the front page of the website and very comfy they are too (though they come up slightly large). I would have bought more but I didn't have the space (or the cash). The website also has a sale section (click on 'Outlet') and seems to have some very reasonable deals. What is annoying is that the HPs I bought in Spain don't seem to be available on any other HP website - UK, US or France! Don't know why this is but it's a pain as I'd love a pair in black as well.
What I found generally is that most Spanish shoe shops only cater for sizes up to 40 or 41 at a push. My 'Tiene usted calzados en talla 43' was met with either amazed or apologetic looks. It's a shame because there are some beautiful leather shoes there and had I had smaller feet I'd have spunked all my holiday cash on shoes!
When I have some more time I'll write about shops in Madrid and Barcelona. In the meantime if you know of any others elsewhere please comment below and I'll add them in.