The best source is another compact. Luckily CRTs of all compacts are electrically compatible, so you can use any compact's CRT in any other compact.
The deflection yokes are not all compatible, however, so they're best left with the unit; swap only the CRT, and you won't have to worry about figuring out whether this yoke goes with that Mac... [if you want to keep the CRT and yoke together, see Scott's sticky on CRTs, above.]
Back in the day, there were also many small monitors (esp. from POS terminals) with electrically compatible CRTs, but many of those were green or amber.