We've replaced several axles for customers (mainly because the stub axle has corroded and in some cases collapsed) and always bought them from the relevant trailer manufacturer, (paid the premium price!). But we came across TrailerTek - Trailer Parts and Spares
a few years ago. Its a one stop shop for all things 'Trailer'. Amongst everything they sell they do a range of bespoke braked and unbraked axles, you just give them the spacing you require (explained on there site) and its made / galvanized / delivered within 4 weeks for approx. We paid approx £300 inc delivery for a 1500kg braked axle. Chris.