You place PHB equipment lists under the purview of the DM? I can't think of a single instance where I even bothered asking the DM about buying a horse, let alone having a DM intervene when I did so. It's in the PHB for a reason.
via Gygax...
1e PhB said:In many campaigns it is necessary for the characters to shop in a number of places in order to obtain everything they desire. A few games will have a trader's establishment in which everything, or nearly everything, can be found, but the average costs will be higher because these traders are middle-men.
As in most dealings, the buyer should always beware. Things may be as they appear or they might be otherwise...
B2 lays out what part of the list the provisioner has, and what part the trader has. There were no horses for sale in the keep as far as I can tell.
2e PhB says the DM may alter the equipment availability or prices but should tell you they've done so (but its good to for the player to check with them if they have doubts).
2e DMG has special rules (running a bit over an entire page long) on horse quality and buying horses because its tricky. Although, "[f]or simplicity, [the DM] can assume all horses are of average quality."
The Black Company setting for 3e lists the rarity level for each type of equipment and has rules on locating it (a light horse is "occasional" and smaller towns may not have one at all times; light war is "specialty" and might have to be specially imported).
PF notes that GMs may wish to restirct availability, but most things shouldn't be that hard to find.
But sure, if "Aurora's Whole Realms Shop Catalogue Counter"s (Volo's Guide 2e) are particularly common in the villages and towns you frequent then the above concerns are probably moot.

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