There are two types of scanners capable of scanning slides (also negatives). The first is very specialized, perfectly suited to this role and very expensive (or cheap and rather — if not very — bad). The best known was the Nikon Coolscan (there were several models). I think one has never done better, at least in the amateur field. But alas it is no longer manufactured. And I understand that the high-end is now selling more expensive used than it was sold new, like a Ferrari! There is scarcely more scanning of this type today, at least with a sufficient quality.
The second type is a conventional flatbed scanner with an accessory for positioning and illuminating slides or strip films. But the resolution of such a device, if it is generally very sufficient for an A4 page, quickly becomes limited for a surface of 24 × 36 mm. The result will generally be disappointing.
One will find an advanced comparative, with serious arguments, and relatively recent (updated 2012) at clubic.com.
Several websites offer the digitization of slides or analog films. Here again, we have a choice between relatively cheap offers but with a low resolution, and others of high quality but very expensive (the fine analysis of a slide can last several minutes, and time is money).
I found an interesting article, although a bit old (2010): Four solutions to digitize your slides. It says, globally, in more detail, the same thing as me. Note: dust / scratch detection / correction does not work for Kodachrome slides. The article talks about four solutions; for the moment, we have only seen three: dedicated scanner, flatbed scanner, online lab. The fourth is to use a digital camera. But what I will expose you is still a little more evolved than placing a cardboard tube in front of the objective, as evoked in the article!