For the Argent Squire, you have to become a champion for each of the Alliance cities in the Argent Tournament. You can only start this at level 77, then it takes I think a month or more of dailies. The hardest part about this is might be getting exalted with each city. You can either do a lot of quests in Azeroth or throw on tabards and grind through dungeons. If you've never played as Alliance, I would suggest doing some quests, mostly Westfall, Redridge, and Southern Barrens, those have some good stories with them.
The Guild Page and Guild Herald can only be purchased by a character in a guild that has unlocked certain guild achievements, Horde Slayer (get an honorable kill on a level 85+ of each Horde class and race) for the Page and Profit Sharing (loot 100,000 gold from creatures) for the Herald. You also need to have certain reputation with the guild to purchase each pet, honored for the Page and revered for the Herald.
Actually, even with a faction change you'll have to grind. A faction change resets all progress with the Argent Tournament, which would make you start over from the beginning. Then you'd also have to work on getting reputation with a guild in order to purchase the guild pets. I suppose if you already had the Horde versions of all three pets, then faction changed, they would all convert to the Alliance versions.
Without a faction change, you're looking at a grind no matter what. I would suggest starting by rolling a Death Knight and joining a guild that has the two guild pets unlocked. Finding the right guild shouldn't be too tough, any guild that's been around for awhile should have them available, they don't take any special effort (maybe Horde Slayer does). Leveling up to 77 and doing the Argent Tournament grind should be more than enough reputation to get both of the guild pets.
Edit: Forgot the requirement to be exalted with each Alliance city to champion them (doh!).