The product I am currently using is Schwarzkopf BC Bonacure Colour Freeze Silver Shampoo its a 250ml bottle which I got for $24.95, so they are little more pricey then your super market shampoo but in my opinion is worth the money. Generally speaking maintaining blonde hair is an expensive process as it is, so spending all that money the extra little bit to maintain in between hair appointments is worth it.
This shampoo is designed to protect the colour by balancing out the pigments to remove any brassy tones in your hair colour. I like to follow this by using a deep conditioning treatment just to repair my hair from any damage caused in between washes. This brand also does have a conditioner available which you can use however I have yet to try it to know how good it is. As this shampoo is a little on the pricey side I use it but if I need to rinse and repeat I tend to use a different shampoo. When using this shampoo I find leaving it in for 5 minutes gives a lot better results, often after using it I am asked if i've had my hair done which always makes me happy to hear.
Here is an image of a before an after shot of my hair using the shampoo to give you an idea of the results and what happened with my hair. The lighting in the photos isn't great but you can see the lightness in tone in the after shot and get the idea of the effect the shampoo has in your hair.