Second Slam (races of eberron)
Is a direct exception that adds a second Slam Attack so long as you Full-Attack and use your Slam once. As the above, we're using one Slam even through we could make two, this exception applies and adds another attack at the price of a single Feat.
Necklace of Natural Weapons (savage species)
You can enchant the Battlefist with a Holy, as it notes it adds extra damage to your weapon and Battlefist doesn't actually deal damage but instead transfers it's bonuses to your Slam/Unarmed weapon(s). Now we buy a +1 Collision Necklace of Natural Weapons for the standard Unarmed Attack, there again transference.
Scorpion Kama (magic item compendium)
And finally, Scorpion Kama. It's a Kama that deals your Unarmed Damage which would include Holy/Collision, but also it add it's own bonuses. Let's just add Blurstrike. So for the price of a +2, +2, and +1 addition which costs 18,000gp we've created an effective +5 weapon. A 32,000 gold cost reduction if you don't factor the raw price of the Kama. Which I think is somewhere around 6k total, so you still save money. Obviously, this lets you afford more Enchantments for the same price.
Cave of Gems (mysteries of moonsea)
Really just a minor bonus. Put a Masterwork weapon near a crystal and it gains a certain Enhancement, for a flat rate 2,000 gold. As a +1 weapon, the Battlefist must be inherently Masterwork. So ransack the area and tack on Flaming, Frost, and Shock for 6,000 gold before you use Graft Weapon.