Not somewhere I would normally frequent, even though I live right next door to them, but Halfords will do an air/con top up while you wait, as long as they are not too busy. A friend took his Kia in when they came to see us last week... I have no idea what they charge. The usually need a top up every three years or so, especially if you don't use it in the winter, as the seals will dry out and maybe let by.
A lot of people never turn their air/con off.