Bach is still the industry standard when it comes to trumpet and flugel horn mouthpieces. Bach flugel horn mouthpieces have the same rim sizes as the Bach trumpet mouthpieces. For example, if you play a Bach 3C, then try a Bach 3CFL flugel to match.

Stork Vacchiano Flugelhorn Mouthpieces

Curry has developed some very nice flugelhorn mouthpieces with a similar rim and fell as the popular trumpet moutpieces. For specs see "more information".