I have two favourite brands. They both sell underwear and swimwear.
The first is the German company, Olaf Benz. I bought my first thongs from this brand, they're really excellent quality.
But I also recommend to check Kriszsartoria, here's the link to the English language website:
http://www.kriszsartoria.co.uk/This web shop allows you to have your own custom swimwear sewn for you, whether you're male or female. You can choose from several ready made models or combine dozens of front and back styles, fabrics and colours. Their products are great and the service is excellent as well. Kriszsartoria has become my main supplier in the last year for thongs and g strings, both for underwear and swimwear!