Look in ebay or such web site for used agile TV modulators. The kind your cable company is getting rid of. Something like this
http://www.ebay.com/itm/BLONDER-TONG...item35d72b1df0 (no affiliation).
And if you're handy with solid state electronics, I've changed the TV channel on various consumer RF modulators. Remove the combo channel 3 and 4 crystal (sometimes called a surface acoustic wave SAW resonator), and for channel 2 use a 18.432MHz crystal (should be 18.41666MHz but it's close enough for a TV set tuner to tune it in, 18.432MHz is pretty common and cheap). for Channel 5, something close to 25.75Mhz should work, and channel 6 is 27.75MHz. You want something within around 300KHz to get the RF modulator reasonably on channel. Be aware that these consumer modulators don't filter the vestigial sideband, so if you have a channel 6 modified modulator, channel 5 will be trashed. So you can only have channel 2, 4, and 6. 3 and 5 will be trashed, and unusable.