RE-supply locally or by pre-shipped box; that is always the question. The answer depends on how flexible you are willing to be. If you need to KNOW that the stuff you need if available, ship it to yourself. If you're willing to settle for whatever's available at the moment, go for the local buy option.
Sierra City can be a hard resupply when the herd goes through. That's a fairly small country grocery store out in the sticks, and while they stock a wide variety of items hikers want, their inventory level of each is also small. You could find the hikers ahead of you have picked it clean. (The same thing happens in Belden, but their store is even smaller and carries fewer items, although there is a hiker box.) The Sierra City PO is right next to the store, so I'd suggest you cover your bases and ship a box there and hope to find some fresh fruit and dairy items at the store.
Going northbound, definitely use Muir Trail Ranch - if its open. They also have a hiker's box and sell every kind of fuel you can imagine, including Esbit tablets. Their ultra-tiny "store" might be able to sell you bandaids, batteries, and lip balm, but NO FOOD.
MTR is about 1.5 days SOUTH of Vermillion Valley (if it's open) so you don't have to carry as far from Kennedy Meadows (or from any intermediate resupply point like Horseshoe Meadows or Onion Valley).
Reds Meadow Resort (if it's open) is only 4 days north of MTR (or 2.5 days from VVR). You can do a full resupply from the resort store, including fuel, and there's a GREAT cafe, hot springs CG, and laundry facility for hikers. If the Natl Park Shuttle is running, you can go into Mammoth Lakes, stay in a warm, dry motel, sleep on a REAL bed with clean sheets, eat REAL food at REAL restaurants, visit the outfitter for gear, etc., and really have a ball
Snow conditions will dictate when MTR, VVR, and Reds are able to open. Either way, you MUST send a box to either MTR or VVR, but not to both.
Most of the near-trail resorts in Oregon have small stores that caters to fishermen. Only the truly desperate would try to resupply out of them. Most resorts will hold packages for you; take maximum advantage of that.
You'll have no problem doing a full resupply at the large grocery store in Cascade Locks. The only things you can't get in CL are freeze-dried meals and stove gas canisters; ship those to yourself via the PO (right across the street from the store). There is a Best Western motel right by the Bridge of the Gods and next door to the Charburger Restaurant, so you can get an expensive room if desired. There are 2 other far less elegant and less expensive motels in town down by the store. Be advised, there's no public transportation in or out of Cascade Locks. If you take the road walk from CL to Panther Creek Campground, you have the option of resupplying at the large grocery and/or PO in Carson, WA - halfway between CL and PCCG. It saves you 7 miles of asphalt with a full pack.