Hard to tell exactly what your preferences are, but if you just want old looking caps, a WYSE set has got to be the cheapest way to go. It can be difficult to match the set to a modern keyboard, though.
If you want classic cherry keys you can try to find old boards on eBay, but the Geekhack classifieds has got to be the easiest option. You just have to be patient.
You can also get some of the Dyesub PBT sets from China. imsto has some sets, there are some Vortex sets. They have similar colorways, but they usually look a bit more modern.