This is a bit of a “teach a man to fish” answer – I want to include all of the resources I used to research this question, as they are frequently useful for this sort of question, even though a couple don’t help with Ride. Thus, this answer may be useful to future users who are searching for help with related questions, even if the OP already is aware of these resources.
This resource is great for finding ways to get various skills in-class.
We note that Ride does not have any particular entries, but also that Player’s Guide to Eberron’s elf-only Aereni Focus, Unearth Arcana’s variant Skill Knowledge, and Dragon vol. 326’s Flexible Mind can each get you a class skill of your choice. Flexible Mind is the one most likely to work for a variety of characters in a variety of games, but Dragon material is always questionable.
Aereni Focus sounds perfect
Elf, region of origin Aerenal, 1st level only
A single skill of your choice is now a class skill for you, and you gain a +3 bonus on checks using that skill.
Aereni Focus counts as Skill Focus for the purpose of meeting any requirement or prerequisite.
This is interesting, since some of the elves from Aerenal – the Tairnadal – are the best horse riders in Eberron. As elves, they are also noted for archery. Best of all, Valenar is just a colony of the Tairnadal elves – who reside primarily in the northern steppe of Aerenal, and Valenar itself consists of similar terrain.
Note that the region of origin requirement is a little strange: Tairnadal children are not born or raised in Valenar. They are born and raised in Aerenal, and only go to Valenar when they are old enough to join a warband. Talk to your DM; the other “region of origin Aerenal” feats would be a little odd for a Tairnadal elf, and there are “region of origin Valenar” feats that, while technically inapplicable to most of the Tairnadal, are clearly meant for them. But Aereni Focus itself fits the Tairnadal as well as it does the usual elves of Aerenal.
But, if that doesn’t work for you...
But it doesn’t help; none of the cleric ACFs give Ride, and the warblade doesn’t have any.
Skilled City-Dweller is an ACF that can actually apply to most classes, swapping one or more skills for other particular skills. In this case, Ride is not on offer, but it is notable that if you already had Ride, Skilled City-Dweller would allow you to trade it for Tumble. Tumble is a far-superior skill, so a reverse-trade should be acceptable. Note that the warblade gets Tumble in-class.
Unfortunately, it only confirms your belief that none grant Ride.
If any useful class dip grants Ride, Able Learner would make it a class skill for life
If there is any class you might like to take a level of, that has Ride in-class, the human-or-doppelganger-only feat Able Learner from Races of Destiny will allow you to continue paying in-class skill points for ranks in it. Since your maximum for a skill is based on whether the skill was ever a class skill, Able Learner effectively makes all class skills you ever have always class skills.
Notable options here include
Barbarian – Rage, or its Ferocity or Whirling Frenzy variants, is fantastic. Fast Movement is kind of useless to a mounted character, but oh well.
Knight – Knights, obviously, are often mounted, and they even get Mounted Combat as a bonus feat at 2nd. Knight is mostly a pretty bad class, but the first four levels are fairly solid, though much better for melee than archery.
Ranger – Obvious choice, since they can do archery natively. The skills aren’t bad, either. Being from the steppe might be a good justification for Favored Enemy (arcanist), an option in Complete Mage which is... well, at least it’s a lot better than other Favored Enemies.
Druid – Saving best for last here a bit. Druid has a ton to recommend it. One, you can get a warhorse as an animal companion, which means there are options like Nature Bond to keep it up, plus you get massive bonuses to Handle Animal with your mount. Second, you can grab a few select spells, which is nice. Dragon vol. 347’s Focused Animal option might be nice; it replaces a lot of things you won’t get with a dip to give you continuous speak with animals and you count two levels higher for your Animal Companion. Races of the Wild has similar options for halflings, but if you’re a human for Able Learner, you’re not a halfling.
If you don’t go with druid, by the way, I recommend Wild Cohort. Depending on level, this may be a better option anyway.
While we’re on the subject, I highly recommend the crusader and warblade substitution levels from this homebrew contest entry. PId6 won with that entry, and I have used it in many games to great success. The desert crusader is a mounted warrior, while the desert warblade is an archer-warblade, and adjusts the maneuvers accordingly. It’s fantastic work, and really fits your character. You may actually want to multiclass the two, if your DM approves them.