Most runs of keycaps for enthusiasts have been made by GMK, EnjoyPBT and by Signature Plastics in DSA and SA profiles. These manufacturers have offered many options for colours and custom legends — which is why there have been so many different variations of them. None of those have offered support for backlighting, however.
You will find many keycaps for backlighting on e.g. eBay. But those are often in thin-walled double-shot ABS or even in laser-ablated paint on ABS, which are of less quality than GMK, EPBT and SA. And support for other layouts than 100% and its subsets are rare.