Any good web sources for antique furniture repair materials?

As noted above, any supplier of upholstery supply items will have basic replacement/repair items as you've listed.

Horse hair and/or hogs hair padding material, as for as I know, can only be gotten from another upholsterer, who would typically salvage these supplies from an old piece, when a customer wants it replaced with newer material. The upholsterer will save this material for restoring another piece with this original material.

In the Boston area and surroundings, old padding material sometimes consisted of marsh reed/marsh grass. For restoration of those pieces, I'm not sure where you would get replacement reed/grass. Dry your own? Marsh grass is noted for its ability to bend, rather than break, during high winds, hence a very flexible, thin (fine) material and excellent for padding material. Don't think straw, broom straw. It isn't straw-like.

Excellsior was another old style padding and is often desirable to keep/replace. It is still available through American Excellsior Company:

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. It's expensive, but the high-end customers have no issue with doing what it takes to restore a piece to its original condition.

Feathers are readily available from just about any upholstery supplier and some fabric outlets, also. Your local fabric shop, sewing shop probably has feathers available, also.

Your local fabric/sewing shop probably has contact with an upholstery supplier, too: Foams, threads, buttons, etc., etc., so some supplies are no doubt available very near to you. There is likely a wholesale upholstery supplier within 100-150 miles of your location and they likely make deliveries to your local upholsterers on a regular basis.

Replacement cabriole and other carved legs are available through a more wood related outlet, but are readily available, also.

Sonny