I don't know if there are downloads of chaptered mp4 files available, but you can easily create one.
All you need is foobar or QAAC:
qaac.exe --concat -o S:\test.mp4 file1.flac file2.flac file3.flac
You'll get a file like that (mediainfo output):
General
Complete name : S:\test.mp4
Format : MPEG-4
Format profile : Apple audio with iTunes info
Codec ID : M4A
File size : 22.0 MiB
Duration : 15mn 15s
Overall bit rate mode : Variable
Overall bit rate : 201 Kbps
Encoded date : UTC 2014-02-06 06:19:56
Tagged date : UTC 2014-02-06 06:20:30
Writing application : qaac 2.33, CoreAudioToolbox 7.9.8.5, AAC-LC Encoder, TVBR q91, Quality 96
Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 15mn 15s
Bit rate mode : Variable
Bit rate : 200 Kbps
Maximum bit rate : 233 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 44.1 KHz
Compression mode : Lossy
Stream size : 21.8 MiB (99%)
Encoded date : UTC 2014-02-06 06:19:56
Tagged date : UTC 2014-02-06 06:20:30
Menu #1
ID : 2
Codec ID : text
Duration : 15mn 15s
Encoded date : UTC 2014-02-06 06:20:30
Tagged date : UTC 2014-02-06 06:20:30
Bit rate mode : CBR
Duration_FirstFrame : -209640
Duration_LastFrame : -16027
Menu #2
00:00:00.047 : File1
00:02:50.914 : File2
00:09:11.421 : File3
Afaik, audiobooks purchased in iTunes come as chaptered m4b files.
.sundance.