We took the risk and had the cubbies built first...I managed to find the baskets in walmart. But it would make sense to find the baskets first if you can. We bought a bookshelf at ikea and bought the baskets to fit there too so at least those worked right off.

I found the nicest baskets at Target, 12x12 inches square, IIRC. Heavy and lined, but mine were brown in white cubbies. Find your baskets first and save yourself the hunt that I had.

I'm with Annie, I'd buy the baskets first. It's much easier that way. There are so many out there of various sizes and materials, wood, wicker,fabric plastic etc.

Annie, How big are your cubbies?

I agree that it might be easier if you were able to choose baskets first...
We have an Ikea cabinet in the basement with cubbies. I've purchased storage boxes from Ikea, West Elm and other sources and all have fit. Dimensions @ 12-12.5" wide x 12-12.5 high and @13" deep.

The opening is 9 3/4" wide by 10 1/2" high. I don't remember talking to the carpenter about the height/width or placement...he decided that all himself as I recall. My baskets were just a tad too wide, but they are canvas on a metal frame, so I was able to slightly bend the frame so they fit perfectly....I like that they have the handles on the front so we can grab and pull them out, like a drawer. Esp given how high they are. DH and I are fairly short vs our carpenter who was 6'4"...

The easiest baskets to find are going to fit the IKEA Expedit/Martha Stewart cubby size which means about 12"x12" baskets, or 13"x13" cubbies.

I'm in agreement with robotropolis... I have the expedit bookcase in basement. The cubby openings are 13x13 as mentioned and 15"deep. There are many basket options for this size.

I agree with pps about the width of the lockers. We have 3 lockers in a 6' space. They are not finished yet but this is how they look in the room. I wanted them wide enough to accommodate a hanging back pack and winter jacket.
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I have found many different types of baskets that have fit what used too be the expedits at IKEA (they have a different name now. I think I would go with similar dimensions to that if I had not found the baskets yet.

I like these baskets (Whitmor Large Woven Strap Tote):

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If you want to hide things - they have held up to daily use for about 2 years so far....

and these from urban outfitters if you dont care about hiding...

My DB built our mud nook. He gave me a rough estimate on what the openings would be, then I found baskets at the Container Store. Once in hand, he was able to finalize the openings.