Car speakers are simpler to buy than home speakers. Only certain ones will fit. Generally, you should use replacements the same size as the ones already in the car, unless you want to drop some serious cash on custom installation. Also, check the depth: if the old speakers are 2.5" deep, new 3" deep ones may not fit.
I used to work in a high-end stereo store that sold car stereos, and I could never tell customers this (because they wouldn’t believe me), but it’s basically true:
Once you have the size right, it’s a good rule of thumb to just use the most expensive speakers you can afford. Up to around $50 per speaker, you generally get sound that everyone can agree is better, by spending more. Just pick a budget and get the best speakers you can afford under it.
But unless you’re a serious tweak-o-phile (true: I have almost always had speakers worth more than my car, and do now), spending over $50 per speaker is probably a waste: you’re unlikely to hear the difference.
If you really want to spend a few extra bucks, get a subwoofer.
And if the store doesn’t want to give you a better deal (they should), they may at least give you free installation. There is absolutely no harm in trying to bargain, even at a big chain store.