As far as Apple displays go, your best bet is the Macintosh Colour Display, this monitor:
Its a beautiful 14" Sony Trinitron fixed at 640x480, they were commonly purchased with LC series Macs back in the early to mid 1990's, so tracking one down shouldn't be too hard, and they have an Apple DB-15 video connector, so it'll plug straight into any Mac up to the Beige G3 with no adaptor (apart from the HDI-45 adaptor that I assume you already have, unless you're using an AV card or NuBus video card). I have a couple of them, and it'd be the perfect monitor for your needs...a lot of the old Mac games from the 1990's look absolutely stunning on it.
Failing that, if you can't find a suitable Apple display, go with a manufacturer who not only makes their own CRTs, but is also known for making good quality CRTs, such as Sony or Mitsubishi. I have a Diamond View (Mitsubishi Electric Australia) 15" CRT, and despite being only 15", and not a DiamondTron, is an absolutely gorgeous display, and supports any res from 640x480 up to 1152x864.