the scout

I wish to convert an old 6 speed racer to a hybrid.

It has a 6 speed band-on downtube changer. Can I get something to fit on the downtube which will provide stops for me to fit MTB type changers on the handlebars? I have seen it done with braze on type changers but not band-on.

Also can I use an 8 or 9 changer as 6 speed seems to be no longer available?

dabac

Depends on the band-on. Some actually use the same mechanical interface to the shifter as the braze-ons, while others use shifters that are pretty much integrated to the clamp. You might want to do a search for "down tube cable stops" and see if something interesting pops up. Otherwise there are a bunch of improvisations you can try, the RD for instance can be easily be run with continuous outer cable and zip ties.

Not really, the spacing is all wrong. One option is to get bar-end shifters, which can be found with a friction setting, and then a set of Paul's Thumbies that'll allow you to mount them on a flat bar.
Another option(doable, workable but with some drawbacks) is to upgrade your rear wheel to 7-spd. 7-spd indexed shifters are still fairly common on ebay.