If you want AC for an orc, fighter and cleric are two of your best options, particularly if you are low on GPs. Clerics is probably the stronger of the two choices IMO, assuming that you focus on beating on things rather than answering every little call for healing during a battle (get some cheap wands of lesser vigor, ideally funded by the party, and heal everyone up after the battle completes).
As others have mentioned, never forget the possibility of fighting defensively. Combat Expertise is another option with that. The trick to using these tactics well is to be able to gage what die roll that your opponent needs to hit you vs. what roll you need to hit them. My rule of thumb is that if they need the lower half of the d20 to hit you or a 20, there's generally not a big benefit for these things. The rationale there being that they're not going to be able to reduce the hits (and your average damage) enough to make up for the penalty. If these modifiers can make the roll they need to hit your AC into a 20 it's definitely worth using. In-between gets a bit fuzzier. But generally, the higher the roll they need is, the greater benefit you gain (in terms of average damage) by pushing it even higher.
Probability is an important tool in understanding the effectiveness of AC.
There are also a couple of armor-related feats in Races of Stone and some new armors that can help with your AC too.
Another really ugly thing that you can do if you are playing an evil cleric: creating undead (spells) and commanding undead (via your ability to rebuke undead). A lot of players grossly underestimate how powerful these can be.
Commanding turns stuff that you encounter into cannon-fodder for your party. Let them charge in and take the hits instead of you. Every hit to a skeleton or whatnot that you control is one less that you have to take. As a bonus, they can lug around and wear all the gear that your enemies were wearing (to protect them, plus also so you can sell it later). Equally good, they can perform actions which are questionable for characters to perform, such as opening the chest that the rogue didn't find a trap on.
Creating undead (make sure that you have the material components with you) can be really nasty too. Usually you want to have the single biggest, nastiest skeleton that you can make with the animate dead spell. Which can be 2x your level in HD. If you kill something big, ugly, and nasty, think very hard about making it into a skeleton to serve you. Send it charging into combat (taking hits and unloading the hurt) while you stand back. At 9th level you can animate stuff of up to 18 HD. And there are a LOT of really nasty skeletons that this opens up
Not helpful if you're interested in playing an orc, but I will throw out another notion if you're serious about wanting AC for a fairly low GP budget: A mounted gnome. A key feat for this is the Titan Fighting feat from Races of Stone, which lets you apply the racial +4 AC bonus vs. giants to any creature larger than you. Tack on the small size, and you're already at +5 higher than what an orc can do. I say mounted, as a riding dog remedies the lower speed and carrying capacity of a gnome, plus it opens up being able to use the Mounted Combat feat chain inside of dungeons, which can help make up for the lower damage for being small. Armor, shield, plus a few lower cost items, and you're sitting at a fairly solid AC, for a fairly low budget.
