These are not really common fittings, and my experience with reducing els is that they are pretty pricey. Much more common to use a standard el, then increase the pipe size with a concentric or eccentric reducer, unless you're really pressed for space.
In any event, you need the type of material, pipe schedule, and the flow. The Crane Technical Paper No. 410 will likely have the info you require.
Are these drip pan els installed on safety valve discharges? If they are, it's not likely an issue anyway. If you're concerned, the manufacturer should be able to supply any info required.