Hey there Jason,
Welcome to planDisney and thanks so much for your question. The Disneyland Resort is also my family's happy place - there really is nowhere else like it.
Lockers are such an underrated resort amenity, and I'm glad you're thinking ahead about them! There are actually a few locker locations spread throughout the Disneyland Resort, so you have some options depending on where you're spending your day.
Inside Disneyland Park, the main locker rental location is to the right as you head down Main Street, U.S.A. just behind Market House. There are also locker locations outside the parks in the esplanade (they're both to the left of each park's entry gates), which is handy if you want to stash something before you even head through the turnstiles. Over in Disney California Adventure, you'll find lockers just past the main entrance on the right.
As for your question about selling out — honestly, lockers at Disneyland don't operate the same way a
Lightning Lane Single Pass or
dining reservation does, where you're prioritizing to secure that spot early. They are first-come, first-served, and availability can definitely get tighter on busy days, but they don't seem to often "sell out" in the way other experiences can. That said, if you're visiting during a peak period, like a holiday weekend, then getting there earlier in the day gives you the best shot at snagging the size you need. They come in various sizing options, and the larger ones tend to go faster.
You can visit the
Locker Rentals page to find current locker details and pricing before your visit so you can plan accordingly. There's also a helpful map that shows you where the lockers are located, and you can also find them
Disneyland Resort mobile app!
Thanks again for your question, and I hope your family has an absolutely magical time at the Happiest Place on Earth!
Have a Magical Day,
Greg