What portable vendors exist in World of Warcraft, and which is the cheapest to attain?

There are multiple different types of mobile vendor, with differing features. Some will only buy items off you, some will repair, and some have an inventory of items that you can purchase. The items they stock depends largely on the expansion they were released in.

Pets

• Guild Page - 300 gold, 8 hour cooldown, requires honoured with your guild to get
• Guild Herald - 500 gold, 4 hour cooldown, requires revered with your guild to get
• Argent Squire/Argent Gruntling - 14 gold 80 silver + 10x Champion's Seal, 4 hour cooldown, requires Argent Pony Bridle in order to act as a vendor (an additional 150 champion's seals)

Mounts

The Reins of the Traveler's Tundra Mammoth is a Wrath of the Lich King mount and will only stock lower level foods and drink, while the Reins of the Grand Expedition Yak has the various Mists of Pandaria goods available for purchase. The Reins of the Mighty Caravan Brutosaur stocks reagents, food, and drink from the Battle for Azeroth expansion.

• Reins of the Traveler's Tundra Mammoth - 20,000 gold
• Reins of the Grand Expedition Yak - 120,000 gold
• Reins of the Mighty Caravan Brutosaur - 5,000,000 gold

Professions

• Field Repair Bot 74A - random drop, requires engineering 300, 10 minute cooldown, repair and purchasing vendor only (cannot buy items from it)
• Field Repair Bot 110G - random drop, requires engineering 325, 10 minute cooldown, repair and purchasing vendor only (cannot buy items from it)
• MOLL-E - taught by engineering trainer, requires Engineering 425, 1 hour cooldown, mailbox only
• Scrapbot - random drop, requires Engineering 425, 15 minute cooldown
• Jeeves - random drop, requires Engineering 450, 1 hour cooldown, vendor and repair

Overall, it depends how your character is set up. If you have engineering, the variety of repair bots at your disposal can provide you with a fairly consistent uptime for repairs and selling items. If you've already got plenty of Argent Tournament dailies completed then the task of getting hold of an Argent Squire is relatively straight forward, and if you're in a guild and have guild reputation, getting hold of the guild page/herald is very easy, but these do not provide repair options.

The mounts, while expensive, provide a full range of options but can only be mounted up outdoors (which generally rules out using them inside instances as these are usually "indoors"). The prices of the mounts scale with reputation, so for example, if you've got yourself to exalted with the Kirin Tor, the Reins of the Traveler's Tundra Mammoth will reduce in price to 16,000 gold without the bartering guild perk and 14,000 gold with the bartering guild perk.

The three versions of Noodle Cart from Cooking also act as a vendor. They are more accessible than the engineering version since they don't have any profession requirements, but each cart can cost a hefty bit (50-200g per cart depending on server).

Also worth mentioning, since patch 6.0 the guild perk "Bartering" is removed and the mammoths cost at minimum 16,000 gold.