You only need a peat (acidic) compost if you have an alkaline soil ie you live in an area where the soil is naturally limely or chalky. Magnolia are quite happy in neutral or acidic soils so other composts will do just as well.

mine is planted in alkaline soil and I feed it with ericaceus plant food twice a year it is doing fine. The beech leaves that also fall on the bed will also help the acidity.