Some indirectly useful (I hope!) answers.
1 - Costco in the UK had a mixture, as they sourced (in most categories) through US brands, their own Kirkland brand, and locally. Like you, I would presume they mostly carry local sizes...
2 - ...but bed sizes are not standardised in Europe - even IKEA, which used to try to force Scandinavian sizes on its customers, has bent to local preferences to a large degree. Bed sizes vary between Spain and Portugal, too (standard double in PT is 190x140, whereas in Spain it's 190x150 - and in Germany, it's 200x140!) My point being, bring the best bed for you, and go from there! We brought a UK superking 200x180, as it's almost impossible to find those here...
(As an anecdote, we've "inherited" beds here (as it's typical for older "inheritance sale" properties to come goodness knows what in them!) and none of them has been quite standard.)
3 ...and finally, you can get mattresses and bedding easily made to size in Portugal, if what you have is not standard! Online, this (www.lusatextil.pt/pt/)is an example (I have not used them, but bookmarked them because of a recommendation at some point from someone), but it's one of many, and local (non-chain) bed, linen or mattress stores will be able to direct you somehere. Or, a retrosaria will adjust your favourites, for a few euros - they're everywhere!
Oh - and also, why not consider IKEA itself if you go with the IKEA bed; or order online from Germany would also be an option, if you feel uncomfortable getting things sorted bespoke initially!
If you DO go to Costco in Seville (or if anyone else has been), I'd love some feedback as to how good they are, compared to US/UK. I prefer small, local shopping, but there's one or two things I found them amazing for in the UK.
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