Everyone has a different store and brand they like, but just find a brand from a store you like. Victoria's Secret is my favorite to buy bras and panties from, but other people might not like it. I just say whichever store you like best, you should keep to it, but maybe get a different size? If you can't find one, you can always try different types of stores that have bras and panties to fit. Sorry if I don't help much. sweatdrop
I don't think I saw anyone mention that some brands carry sizes online that they don't normally carry in shops. It's helpful when you're a less common size. Half-cup sizes are really nice too if you happen to be a bit lopsided like I am. My problem is that I'll find a bra that is super comfortable and fits just right only to have the shop quit carrying it.
With panties, I usually just try a bunch until I find a brand/style/size that is comfortable and then stock up in case they change something. I just went back to buy more panties that fit perfectly last time I bought them but now they're slightly too tight. My old ones still fit fine but the new ones feel like they're made out of a different material. Why can't they just stick with what I like? /: