How to make wood slats of slightly different lengths, all of the same length?
I think that you need a circular saw. You could probably pick up a high-quality, used one from a pawn shop for $20 to $40, or a cheap-new one for $40 or $50. Usually, circular saws and sanders aren't for rent... and a new saw is probably cheaper than renting a tool. And if you want to return it, I'd bet the pawn shop will give you a few bucks (back) for it. Alternatively, you might find a hand saw for under $20.

Blade thickness isn't really relevant here, make a mark with a pencil and a straight edge (at 29.5") but don't cut the line... cut adjacent to the line so that the pencil mark is still touching the corner of the board after your cut. 

Buy some sandpaper anyway... for the splinters.