I was wondering if an old tv from the 60s like the Sony TV-900U works with an Apple 1?
Pictures at http://picclick.ca/Vintage-Sony-Solid-State-Transistor-Portable-TV-Model-201680878754.html
Yes, as long as you can connect an RF-modulator like the Sup'r-Mod to serve as the antenna.
Any pictures or guide on how to do this?
Sorry just noticed this posting.
I don't recall if the Sup'r-Mod needs a -5V which isn't on the Apple-1 connector. With an Apple-1 you need a modulator that only needs a +12V and/or +5v. You can also just get any external video modulator. Just search on eBay for Vintage Video Modulator and get a radioshack one. It will be self powered. Won't be date correct but will work.
Another option is to look for a 1970's security monitor, though they tend to be expensive. You used to be able to get a vm4209 monitor for $50 to $100, now I have actually seen good examples change hands for $500 to $1000. You can also keep on the look out for factory TVs with video inputs. Very rare for the 70's, but they did exist.
Or finally find a TV shop guy who can modify an old TV to have a video input. This is what most hardware hackers would have done back in the day. Remember TVs even came with schematics back then.
you can also get an older zenith or what ever (they where all the same in the 70's and 60's) tv and unplug the tuner, The tuner as I recall only outputs composite, so it should be plug and play.
My first monitor was a small 9" B&W Sony that ran AC or batteries.
I added a video jack and a switch on the back so I could use TV or Video.
That little switch came in handy when making disk copies with LockSmith.
If you don't care about it looking vintage you can still buy RF Modulators. Here's one at Target, but a Google search will turn up plenty of options.
(Edit: Note that a modern RF modulator will have a 75ohm coaxial output, you will of course need a matching transformer to connect to the antenna screws on a TV made before, heck, probably the mid-80's.)