If you are modelling in HO all the OO products on Scalescenes.com can easily be resized down to HO scale (other scales see below).
If you are printing on to A4 or Letter, simply enter 87% into your printer's 'Scaling' options, this usually found in the 'Advanced', 'Properties' or 'Options' section of your print window. Consult your printers 'Help Index' for more details.
TT (1:120) - reduce OO 63%
TT (3mm) - reduce OO 75%
S - enlarge OO 118%
Z - reduce N 67%
N (1:160) - reduce N 93%
T (1:450) - reduce N 33%
Card thicknesses for HO and N (1:160 US and European) can remain as recommended in the instructions.