Flowers I don't know..but technique I know. Just collect the nectar wearing sterile gloves and inoculate in Sabouraud's dextrose agar, Brain Heart Infusion agar and Potato dextrose agar..

What is the general criteria used for selecting flower samples that might have the possibility of carrying yeast?

maybe bad smell or discolouration.

do you think that we can isolate yeast from nectar ?

It's possible..you can try and see

We tried doing it but couldn't get any satisfactory results. I think we can try using a discolored or bad odor flower for isolation purpose

you can also try using Sabouraud dextrose broth or Soyabean casein digest broth and incubate at 25 C for 3-5 days then streak on Sabouraud's dextrose agar or Potato dextrose agar, incubate at 25 C for 3-5 days to observe yeast/ fungi growth. Using broth may help mixing of nector as well as good for enrichment.

Dont know much about flower selection. But try random sample collection as well as as Sayan suggested discolored or bad odor flowers.

Thanks....

I will soon record my response :)