I don't have experience with maps outside of Canada and the USA so hopefully others will comment. I'd still recommend looking at Garmin (link
) as they're bigger than all the rest combined and a result have a huge user base which have developed their own custom maps. If Garmin itself doesn't cover the areas you're interested in it's likely someone will have made a custom map in Garmins format.
Generally each GPS manufacturer has it's own proprietary map format and cannot upload maps from other sources.