If there are no existing questionnaire that seems to adress the types of atitudes you are interested in, I suggest adapting attitudes towards any minority group by replacing the group of interest.
Agree with Jan Antfolk , its a good strategy to start off by going through available scales on attitude towards any minority group, particularly if you can find a scale on minority in the same country, which you are targeting for your study. Further, you can go through available literature on transgenders and incorporate relevant issues within available scales on minorities.