The best way to find the kind of flowers bees like is to go shopping on a sunny day. Wild bumble bees, honey bees and mason bees will be at the flowers that produce nectar.
There is an old variety of red heather that is very attractive to bees because it produces nectar. I think it is called Kramer red. If someone can confirm the attractiveness of this variety of Heather- that would be great.
Of course the shrub Pieris japonica is a great source of nectar too.