The basics of most uniforms - shirts, trousers, tunics and jumpers - are worth investing in, especially if you have more than one child. Cotton mix tunics tend to wash better and last longer the nylon versions - they are less likely to become tatty and so can be passed on to another child. Likewise with shirts: pay attention to collars and cuffs - it is often worth applying stain remover to these areas before washing to help maximize the life of these garments. If your school doesn't have a regulation jumper or cardigan, seek out good quality versions that will wash well. Feeling comfortable and confident in their uniform will also help your child to be a happy learner.
Many schools have secondhand sales but even if yours doesn't, you can always get together with other parents and informally swap and share clothes you no longer need. Expensive items like blazers are worth looking after well so they can be passed on.