Where can I find congressional votes on old legislation?

[Unconditional Surrender Truman]

This is only a guess, but prior to 1807 New England merchants were deeply invested in the Atlantic slave trade and were resultantly extremely reluctant to abolish what was a lucrative source of revenue for themselves and their economies. In Jefferson's account of the negotiations over his draft Declaration of Independence, he places the blame for the deletion of a clause condemning the slave trade on South Carolina, but it probably elicited equally strong opposition from the New England states who did not want to give up the profits from the traffic of slaves even if the institution itself was less essential to the Yankee economy. Meanwhile, slave states in the Western interior like Tennessee and Kentucky were more willing to endorse abolition of the trade, as they lacked seaports to which African captives could be imported and thus were reliant on the internal slave trade via New Orleans and the Ohio and Mississippi rivers.

As such, while it has sometimes been overlooked, the abolition of the Atlantic slave trade was a very big deal in that it eliminated (to a large degree) the financial incentive for the Yankee merchant class to support the continuation of slavery. In spite of this, reliance of New England textile mills on Southern cotton through the first half of the nineteenth century (as well as Northern reliance on such products as sugar and coffee that were also produced by slave labor) meant that Northern manufacturers were still very hesitant to endorse a frontal attack on slavery —that is until the changing economic environment of the 1850s made slavery a (perceived) greater threat to business than the loss of Southern raw materials.

It is surprising coming from JQA, given his later strident opposition to slavery, but we must remember that this was relatively early in his career when he was still hoping to be reelected to the Senate. Like many ex-presidents, he felt considerably freer to speak his mind and go against the political consensus after leaving the White House, when his service in the House of Representatives was essentially a favor to his constituents and not a stepping stone to higher and greater things.

[Unconditional Surrender Truman]

For anyone interested in a visual representation of the geography of the slave trade debate, I have made this map of the Senate vote.

In all, five slave state senators voted in favor of passage and four free state senators voted against. (Connecticut, New Jersey, New York, Pennsylvania, and Rhode Island among the free states still permitted those born before passed of their manumission acts to be held as slaves—New Jersey, in fact, had only adopted it plan of gradual emancipation the previous year.)

There were only five Nays in the House: Silas Betton of New Hampshire, Martin Chittendale of Massachusetts, James Garnett and Abram Trigg of Virginia, and David Williams of South Carolina.

[Unconditional Surrender Truman]

Found this in a very old dissertation (c. 1973) titled "John Quincy Adams and Slavery."

Included with the Louisiana territorial bill was an amendment prohibiting the importation of slaves into the territory from another state except by a citizen of the United States, and also prohibiting the importation of slaves into the territory from any port in the territory or through any state allowing importation. Although he believed slavery was morally wrong, it did have its uses in connection with commerce. He believed that the amendment's provisions for the prevention of importation of slaves were insufficient, and he refused to accept the constitutional right of Congress to legislate for the territory. Later on, when another bill was placed before the Senate to prohibit the slave-trade as soon as constitutionally possible, Adams objected once more on constitutional grounds, contending that such a law could not be passed until after 1808. Adams' refusal to accept the Louisiana territorial bill and the prohibition of the slave trade before 1808 were predictable for he was a strict constructionist of the Constitution and remained so all of his life. In later life he was to once more come upon the slavery problem in relation to the right of petition. Even then, while believing that slavery was wrong, he denied the right of Congress to abolish slavery, believing that it was not part of its constitutional powers.

I do not know if this is interpretation in keeping with more recent scholarship on JQA, but it is an interesting perspective on an admittedly perplexing vote in 1805.

(For reference, here is the amendment to the Louisiana territorial bill cited above.)