I just replaced all of my brake pads. The front was $25 for all four, the rear was around the same price, but I had to buy the clips separate for another $6 or so. Advanced Autoparts and O'Rilleys both carry them. For the most part, especially for mechanical stuff, big box autoparts stores will have 99% of what you need. I did go through isuzupartscenter.com for new weather strip and clips. Yes, the prices are high ($75 for the strip and $40 for the clips after shipping) and it takes around 3-6 weeks for delivery, but they really can get anything. If you ever need a front, inner, wheel bearing, NAPA autoparts is the only source that I have found to be reliable, but it must be pressed in by a shop.

tl;dr Just about anything mechanical can be found online or in large chain parts stores, anything cosmetic you can check for used parts here or on FB, or pay the price from isuzu.