If you happen to luck into them, you might also look at the older Vivitar sets. Vivitar sold two versions of a three-part set, with 12, 20, and 36 mm. tubes. Unlike current ones, the old ones were nicely made and all metal. I have two sets, both of which work well. The( Pre-AI) AT-3 won't take an AI lens on the 12 mm. ring, but will on the others. The skirt safely allows mounting on an AI camera, though of course you must stop down meter. The AT-3AI works fine on all cameras but can't readily accept rabbit ears, so stop down metering on a pre-AI camera. Both have nice smooth automatic aperture mechanisms, and I have routinely used the full 68 mm. on my 200/4Q lens for bug chasing, with no ill effects.
If you can get the Nikon ones cheaply enough, and especially if the PK 13 is all you need, I wouldn't bother to hunt too hard for the Vivitars, but if you bump into them they're worth a look.