There may be various oils etc you can buy, but in reality, there is no real way of rapidly growing your hair. The best thing to do is just ensure you look after it the best you can. This way, when you do reach your required length, it will be in good condition and easier to manage and maintain. Alternatively, have you considered hair extensions. You would need to find a reputable extensionist, but this is a good way to get the desired length and volume instantly whilst you are growing your hair out. If you don't like the idea of fixed hair extensions, there are all kinds of clip in hair extensions that you can put in and take out as often as you like within a few minutes.
Hope that helps!
Tracey