We are planning to stabilize the laser (Solid state diode laser) using PDH technique. Our required FSR is 1GHz and finesse is around 2000. So we are looking for dielectric mirrors in reflectivity of the order of 99.98.
Thy have pretty big choice and I am sure you can satisfy even very fancy requirements. We have ordered from them 1inch concave mirrors with reflectivity above 98% for 780nm (for cavity).
That's pretty specific, I don't think you can do that using the standard online design tools that some companies offer. Actually, the high reflectivity and the wide range are two contradicting parameters (the higher the reflectivity, the narrower the wavelength range). I'm not an expert in this, but 100 nm and 99.98% to me sounds pretty much at the edge of what you can do. Maybe, one can use non-standard coating materials with large \Delta n between H and L. Contact Layertec (www.layertec.com) and ask them to design that for you.
they almost always accept orders for non-stock optics and do things really well. I think they should have glass base with such curvature. Coatings needed. This may take few weeks because they have quite a lot of orders. But it definitely worth to contact them by e-mail.