You don´t need a "special Fluke calibrator". A stable voltage source, checked with an other (calibrated?) DMM is good enough for a 15B+.
Maybe you'd like to walk us through the procedure after reading that document.
I've got a super-stable voltage source. It's connected to the meter. Now what...?
OK, no problem:
1. remove the calibration sticker
2. connect a external 3 V battery to the battery contacts
3. turn the rotary switch to "VDC" (if you wish to recalibrate VDC, of course)
4. connect the meter to your "super-stable voltage source" with exactly 3.000 VDC and wait a few seconds to stabilize
5. short on the PCB the contacts "CAL1" and "WP6" (there are under the cal-sticker you have removed) until you hear a short "beep" and the high voltage mark appears on the display
6. push the [HOLD]-button to confirm
7. turn off the multimeter
That´s it!
Repeat this steps for VAC (or other functions, see table 7 in the calibration manual) similar.
It works fine on my 15B+