Hey Chopper,
I just ordered those rear door screws from St Charles Isuzu. If you call and ask for Merlin, he's well versed in those specific parts. The screws are sold individually (something like $3 or $4 each) and then the plastic receptacles are sold in a package of 4 ($4.75 or so for all 4). Be sure to ask for the specific color screw! I ordered red, but they also come in black and gray.
You could buy a replacement set of four screws with four receptacles for around $20, shipped free!!!! Always nice to have spares around.
Oh, and a few people have already done your insert-dowel-into-screw mod. It IS a cheap way to prevent breakage.
PS: Here ya go: 1-800-727-8066 St. Charles Isuzu - ask for Merlin
OEM roof rack, Thule Xpedition 696 rack, Rancho RS9000 shocks, OME 913 coil springs, 3" lift, American Racing Teflon Chambers wheels, Yokohama Geolandar HT/S G051 tires (275/70/16), Energy Suspension greaseable bushings, steel-braided Kevlar-reinforced brake lines, WeatherTech wind deflectors, Malibu 18w driving lights, Plasma SuperWhite 100w headlights, NGK Iridium plugs, K&N drop-in air filter, Optima Red Top, Psychos2 rear-door subwoofer enclosure w/CDT Audio 10" sub, Blaupunkt subwoofer amp, iPod Video 80GB, Kenwood KVT-617 DVD head unit w/7" motorized touch-screen, Kenwood GPS HD navigation module, Kenwood iPod adapter, Kenwood back-up camera, Sirius radio, Flowmaster Extreme Off-Road muffler (black diamond-plate steel) w/ carbon fiber tip, Ford Taurus OEM cup-holder/coin tray (single-DIN/lower bay), personalized license plate "ORBITAL", 3M window tint, debadged, removed rear seat, custom 'radiation symbol' side magnetic signs, VX Concepts front skid plate