PPL Motorhomes is my go-to vendor for parts that are specific to RVs, but not to brand. For Winnebago-specific parts for my coach, I call Lichtsinn Motors. For items for my RV that are not necessarily RV-specific (like LED bulbs), I use Amazon. My wife has an Amazon Prime subscription, so I'll probably start shopping for more stuff at Amazon.
PPL has most everything I need for the RV, so good selection. Regarding prices, you just have to shop around and it depends on the item.