John Frieda has a range called 'Volume', that you can get in Boots/Superdrug etc - I have used a spray in the range called 'Thickening Blow Dry Lotion', which you spray on to towel dried hair after washing. It's pretty good, worth a try to see if you like it - about �4-5 from memory. Once you're ready to blow dry, tip your head upside down for extra body too.
KMS do a mouse and a spritzer type, that is fab, you have to get it from Hairdressers, not cheap at around �10, but it does work, i have long hair and it gives great lift at the roots which make you feel your hair is thicker.
Try Matrix biolage range.you can get it on-line or reputable salons, not in shops as it is for professional use only, not cheap, but really concentrated so econominal to use. Bodifying/volumatherapy is new and really good.