Many of these recordings were part of the now-defunct WDW Forever system, so these CDs show up on eBay from time to time. You may also want to keep an eye on the newsgroups. And of course, if the Wonderland system ever does come back to WDW, these might be included.
It should be noted that the Forever system sold all but one of the recordings played in the loop -- and that the "missing" tune was actually on the kiosk hard drives but never offered for sale, which is why it, too, is available.
Furthermore, the guitarist in question is Laurence Juber, a talented musician who is known popularly for his membership in the final (1978-1980) incarnation of Paul McCartney's "other" band, "Wings".
If Laurence's work interests you, you'll be happy to know that he has released several CDs over the years in the same solo acoustic stylings he's utilized not only on the Character Grove BGM, but also on several other 1990s-era recordings for the Disney theme parks.
The "missing" tune is the 14th track in the loop, "Give A Little Whistle".
The actual running order of the loop goes like this:
01 - Mickey Mouse March
02 - Minnie's Yoo Hoo
03 - Never Smile At A Crocodile
04 - Trust In Me
05 - Ugly Bug Ball
06 - The Walrus And The Carpenter
07 - See An Elephant Fly
08 - Who's Afraid Of The Big Bad Wolf
09 - The Wonderful Thing About Tiggers
10 - Winnie The Pooh
11 - Pink Elephants On Parade
12 - Bare Necessities
13 - I Just Can't Wait To Be King
14 - Give A Little Whistle*
15 - Feed The Birds
16 - Heffalumps And Woozles
17 - I Wanna Be Like You
18 - Little Black Rain Cloud
19 - Reluctant Dragon