Γεια σου Ιάκοβε
"θα ήθελα" is more polite than just "θέλω", so I would suggest you use it. When you are not a native speaker it is better to use "safe" stractures and not informal or colloquial forms.
As for "Μήπως έχετε" is acceptable, even if you are sure that they have the item. In this case, though, it is better to say "Θα ήθελα".