See
https://www.bbastrodesigns.com/diagonal.htm for secondary sizing.
Note distance from diagonal to focal plane is semi-diameter of the tube plus focuser height plus allow 25mm back focus distance. This will affect secondary size along with how much edge ilumination (magnitude drop) you are prepared to accept and that depends on your observing preferences.
I.e. If you are variable star observer you will be using a low power eyepiece for maximum field and you will want 100% illumination (zero magnitude drop) across the entire field stop diameter of the eyepiece you intend on using. This results in a larger secondary.
If you are a planetary observer you can afford to have a small fully illuminated field resulting in a smaller secondary.
For general purpose observing aim for a magnitude drop of about 0.4, I think this equates to about 70% illumination at the edge of field. (Mel used to talk in % illumination but switched to magnitude drop a few years ago; I'm more used to % illumination).
Anyway I digress here but as you can see one can customise and tweak a Newtonian telescope in the early design stage depending on what interests you.
As for secondary mirror suppliers, try
http://www.antaresoptics.com/SEMirrors.php
Buy the highest Wave PV quality you can afford as any difference will show itself under the best seeing conditions. However any of the mirrors offered are first class.
Note don't undersize your secondary too much as you will have a lip around the edge if using a proper holder (if glued you have no lip but I don't like that method of mounting and don't recommend it). Also the edge of a secondary may or may not be as flat as the centre so any lip will cover that.
You will hear advice on reducing the secondary for better contrast and yes this is true but often over stated. Anything under 25% (secondary to primary diameter) is fine, around 20% better and preferred and anything less you really start to lose too much edge illumination. (If planetary only observer on axis it doesn't matter so much).
If you change from f6 to f7 you may or may not be able to reduce the secondary. (I wouldn't bother). Plug in the numbers into Mel's calculator and see for yourself. Use inches as it's easier and multiply by 25.4 to get mm later when needed. (Sec mirrors are sold in inches).
For f6 change first three lines to 8, 48, 7.5 and last line to 0.6 and update.
For f7 change first three lines to 8, 56, 7.5 and the last to 0.6 and update.
If you leave the last at 0.4 it won't offer you the smaller (planetary only) diagonal as an option. The 7.5 is half tube widith plus focuser height (fully racked in) plus one inch back focus. (Change to suit what equipment you have).
The 1.4" is the field stop diameter of the eyepiece you are designing for. This is the only figure you need to multiply by 25.4 and match up against field stop data of an eyepiece. 1.4" equates to approx 35mm and this would fit say a 35mm Panoptic or 21mm Ethos. If using an eyepiece with such a field stop you decide then how much magnitude drop you are willing to accept. (Note I use Tele Vue data because it is readily available; and they are fine eyepieces).
The scale at the bottom of the graph on Mel's page is the field stop radius left and right of centre.
See
http://www.televue.com/engine/TV3b_page.asp?id=214 for field stop data.
If you're happy with say a one inch (25.4mm) fully illuminated field then change the 1.4 to 1.
Note also a faster focal ratio has a shallower fall off and a slower focal ratio has a steeper fall off of light.
If you do decide to change your secondary mirror you will more than likely need a new holder and maybe even a spider (or drill the hole out if the new holder has a larger diameter shaft). Note too your existing mirror is most likely a metric size if out of China. These are not simply the imperial size x 25.4. I.e. If you divide the metric mirror size by 25.4 you won't get the imperial equivalent. If this is the case go the larger especially if getting a new holder with a lip. Last thing you want is a very small fully illuminated field.
Anyway lots there for you, hope it's not too confusing.