Large Moving Boxes come in all sorts of shapes and sizes. If you are looking via the search engines you get the normal affiliates of Smart Pack like Moving Day Boxes, Move Out, Box Quest, and The Moving Box Guy. Their Large box is 20x20x15, comes in increments of 12, and cost $43.82. The Cubic Feet of these Larges are a little smaller than ours, but the biggest difference is they only offer 12 in a bundle versus the 15 Uboxes has for $44.50. That is 23% higher per box plus their Large Moving Boxes are smaller.
Public Storage offers ten 18x18x24 large boxes for $34.50 which is about $51.75 per 15 so although they are a little larger than ours you still pay more. The biggest downside with this company is they don't deliver so keep an eye on gas prices to see if you can afford a trip to the store. Also, bring a truck if you need a lot of moving supplies.
Uline has a Large box that is 18x18x24 for $1.67 each plus S&H. A bundle of 15 runs around $42-$44 after shipping, and if you live in AL, CA, FL, GA, IA, IL, MN, NJ, NY, PA, TX, & WI you'll pay sales tax. It's a little taller than the Uboxes Large and will work better for items that are 20" to 24" tall. If you need a large box that is a little more square then you'll want to stick with ours.
Uhaul also has a large which is 18x18x24 but it runs $3.00 plus shipping and handling if purchased individually. If you buy the bundle of 15 they are $45 plus sales tax.
USA Box Co's large boxes are 18x18x16, which is an unusual size for a large moving box because most companies sell this as their Medium Moving box. So it's quite a bit smaller than the typical "Large" and at $54.99 for 20 a little pricey. Plus if you do not order $99 worth of moving supplies you have to pay shipping.