harry potter - Where do students who don't get into Hogwarts go?

What if they can’t afford to go?

I don’t recall any mention of school fees in the books. With regard to school supplies, see Dumbledore’s visit to a young Tom Riddle:

“That is easily remedied,” said Dumbledore, drawing a leather money-pouch from his pocket. “There is a fund at Hogwarts for those who require assistance to buy books and robes. You might have to buy some of your spellbooks and so on secondhand, but—”

— Half-Blood Prince, chapter 13 (The Secret Riddle)

JK Rowling later confirmed on Twitter that Hogwarts has no tuition fee:

There's no tuition fee! The Ministry of Magic covers the cost of all magical education!

— @jk_rowling on Twitter (July 2015)

What if they don’t get in?

As for students who “don’t get in” or choose not to go, their parents can educate them at home or abroad, if they so wish. We learn that these are possible when Lupin explains the changes at Hogwarts in the final book:

“Attendance is now compulsory for every young witch and wizard,” he replied. “That was announced yesterday. It’s a change, because it was never obligatory before. Of course, nearly every witch and wizard in Britain has been educated at Hogwarts, but their parents had the right to teach them at home or send them abroad if they preferred.”

— Deathly Hallows, chapter 11 (The Bribe)

However, it should be noted that anybody who doesn’t “get in” may not have much to gain. Part of Pottermore entitled The Quill of Acceptance and the Book of Admittance describe how students are entered for the school. If you trust the Quill and Book as accurate (as the article seems to do), then it explains that it keeps out people without any magic:

In fact, the Book's sternness has a purpose: its track record in keeping Squibs out of Hogwarts is perfect. Non-magic children born to witches and wizards occasionally have some small, residual aura of magic about them due to their parents, but once their parents magic has worn off them it becomes clear that they will never have the ability to perform spells. The Quill's sensitivity, coupled with the Book's implacability, have never yet made a mistake.

If you assume that it really can list every magical child in Britain, then anybody who doesn’t get in is either a Muggle or a Squib, and so won’t really benefit from education elsewhere.