The only other US fire engine model I know of is Revell's 1/32 scale Mack CF but this only gets you to the 1980's, it might be a better base than the AMT ALF kits though since the mack is a square cab like most of todays fire apparatus, on the downside most of the aftermarkets stuff is for 1/25 scale.
Some of the places I've found for parts
Model Truck and Fire Apparatus
http://www.mtfaco.com/
Don't let the very basic site and no online catalog turn you off, they have a good selection of metal and resin cast parts, wheels, tools, sirens etc, its worth ordering the catalog.
EVM resin parts
http://www.evmresinparts.com/
Good online catalog, I've ordered from here but haven't received it yet so can't verify the quality, but they have a nice selection of stuff.
Spaulding Trading and Shipping
http://www.stsmodeltrucks.com/
Mostly stuff for big rigs but there is some for emergency vehicles, they have the best set of slam locks (door latches) I've seen, a Photo etch set of 40 (20 square, 20 round) runs about $5.
Chimneyville
http://www.chimneyvilledecals.com/
Decals and lightbars
Like lizardqing said its a rather neglected subject, so you will need to scratchbuild a good bit of the model but I've found its not as hard as it seems like it would be.