Press Q to go into quick mask mode the press D to get default colours of black and white. Next select a round hard brush, Photoshop should show the size of the brush (if not go to preferences and sort it out) now using left and right bracket keys change the size of the brush to match the size of your bubbles dab on the image one mouse click and a red disc will cover you bubble do all the bubbles. Next press Q again to get a selection, that you may have to invert to select just the bubbles and not the rest of the image.
Hope this makes sense I am off to bed now, let me know how you get on.
Regards Rod.