There are in fact 4 different 28" tyre sizes, which coincide with 4 different 700 sizes (700, 700a, 700b & 700c). The largest of these is no longer available.
The only reliable way to match up a 28" tyre is by using the ETRTO number (European Tyre & Rim Technical Organisation). The first number refers to the width, and is not critical. The second number refers to the bead seat diameter, and this must be matched accurately. On this page we have ETRTO diameters of 642mm & 635mm. The more common 622mm is more commonly known as 700c; please see our 700c Hybrid Tyres page.