Did some research.
Toyotas OEM Wipers for the alphards used blades provided by nippons blade division called NWB (nippon wiper blades)
For the 10 series, the fronts are 26"(650mm) and 16"(400mm), with rear being 16"(400mm)
For the 20 series, the rear is the same, but the fronts are 30"(750mm)and 14"(350mm)
NWB blades are coded D** (** is the 1st 2 digits of the size in mm, E.g. D75 = 750mm) and the rear is GRB40
The 'D' is their design range that use graphite coated rubber.
You can buy their HDA or HDB range (the frames are identical, but the rubber blade is coated in varying qualities of water repellant material that coats the glass (better performance with frost/snow))
You CAN buy just the rubber inserts (check that your existing blades are the nippons ones, they should have a metal clip/rail that needs to be removed and reused - the rubbers do not come with this metal part)
I bought a set from amazon.co.jp (you can create an account in English)
Search term for 10 series is NWB D65 D40 GRB40
Search term for 20 series is NWB D75 D35 GRB40
You can also buy DENSO, which is a decent OEM used by Toyota.
Their largest blade, unfortunately, is only 700mm(28") so if you have a 20 series, you will have slightly less coverage on drivers side.
DENSO codes are
DU-070R (28") or DUR-065R (26") driver side
DU-040(16") or DU-35(14") passenger side
And DRB-040 rear.
For all these wipers, the passenger and rears are about £8 to £12.
The drivers side, being much larger is about £22 to £26
These are hook fixings, so any cheapo blade of manufacturers wipers will work if you get the sizes right.
The rear blade has a different connector design, so probably best to stick with the NWB or DENSO designs.
Opieoils (linked in a previous post) is very competitive of you are buying the denso blades, but you will be limited to a mac size of 28"
If you still have the original NWB (Nippon) blades fitted, you can save money by buying the rubbers only, from Toyota, or amazon.co.jp