Bitter Wizard can do it, but malic acid is better. Malic acid is used in wine making and adds a 'smooth consistent sourness'. PA sell it on their web site in the USA at 20% solution or, you can get it in powder form (it's a fine crystal) from good home wine making providers and mix your own in PG or alcohol solution. The only caution is - malic acid cracks polycarbonate tanks.
What are you mixing Andy? A little Ethyl Maltol can bring out the sourness in things like Inawera Grapefruit and PA Apple Green; adding a little enables it as a flavour enhancer - add to much and it sweetens too. If you just want a little sourness Bitter Wizard works at about 2-3% and is available from Liberty Flights - as is EM. On which if you want a citrusy sourness Inawera's Grapefruit is a good way to add this - but it's a strong flavour so be careful.