I have found a couple of places that stock 451 rims. I found this first address on a bike forums thread about twenties from a guy called chambo:

C M Grange tel:01279 758718 this is a wheel builder who can supply wheel chair rims in 451, either 28 or 36 hole. They are something like £11 each plus postage and he is also in the process of having cotterless axles for twenty/moulton mark 3 b/b's made up. Not on the internet, telephone only.

Custom riders internet BMX shop do 451 rims in 1 1/8th sizes though they are pricey at about £30 plus post each. Similarly Alan's BMX on the net do 1 1/8th rims for about 25 quid each plus postage.

Did anyone know that Tioga makers of the lovely comp pool 406 tyre also do 451 versions of their racing BMX tyres?- these are called comp pool 3's and I think compX ie old school mountain bike pattern. Alan's BMX do these as do Wiggle. I am in the process of building an offroading folding twenty(!) and discovered this just a little to late.
Doug
Scotland