Looking on the PCTA website was both helpful and not. When would one start out from the Mexican border. What towns are close to the trail for shopping/mail drops? How critical are mail drops anyways? Some claim that one can do the entire walk just by restocking supplies.
Your best bet is to read some of the trail journals on the PCTA site. They will give you some details of the day to day realities. I’ve got two friends on the PCT right now and they did their planning via the online sites and message boards.
If you have unlimited funds you can shop along the trail, but if you want anything specific you will probably be out of luck. Mail drops allow you more variety in food, the ability to swap gear out and in as needed, and give your hike some structure. I’m sure you can do the entire thing by restocking as needed, but you’ll be at the whim of local stores.
I would highly advise investing in a guidebook; this will get you maps, descriptions of the trail, more-or-less current locations and addresses for resupply drops, probably a few pages on trip planning, and more. If the $30 for a guidebook is prohibitive, you’re probably not financially ready for a through-hike anyway.
BTW, the guidebook I had (for a two-week trek) strongly recommended against trying to do the entire trail in one season, as it required averaging near 20 miles a day for many months.