You can always get the mfr to replace it, but that tends to be more expensive than just picking up a generic 2.5 inch HD. Whether you can actually use a generic 2.5 inch HD depends on the brand, and possibly also the model. The last Gateway I had (which is ancient now) had a proprietary connector. A friend's IBM was standard, another friend's Sony was not. These days even the big mfrs tend to farm out the work (some Gateway and Dells, for example, are re-branded Samsungs) and those tend to be standard and it seems like that's the trend but I can't say that for sure. For any model I was considering buying I would verify this, either by checking a review (good reviews mention this -- look at the sites associated with real physical magazines) or by physically opening the thing up (if you're buying retail). If you're buying mail-order you could try calling or emailing them and demand an answer; after all, you're a potential customer and a sale to you is contingent on it. As for Dell specifically, perhaps someone who has one can tell you (for their particular model, anyway).
Also note that 2.5" HDs come in various thicknesses. Some of the thinner laptops can't accomodate the thicker (higher capacity) HDs even if they have generic connectors.