I have ones branded Meike (can't find the RF version now though), but if you just search for "RF extension tubes" all kinds show up. Anything that has good reviews and isn't the cheapest should be fine.
I highly recommend the Raynox DCR-250 close-up attachment though. I use it in combination with the 100mm macro lens, but it will work with other lenses as well. Most macro photographers prefer that to the tubes. I've had some time to play with both, and I lean towards the attachment as well. Tubes can be advantageous allowing a larger working distance, but I think they produce very soft images when focused to closest point. Close-up attachment allows greater magnification at the closest distance while keeping the picture sharp.
The tubes are cheap enough to get them just to try them and see what works for you though.
