Find subcontractors that already make similar products and get in touch with them. If you order enough, then you'll get a good price, and maybe put your logo on the parts.
There already are companies that make grille blocks ("grille winter covers") for certain models. For example, click on the following image for Skoda covers (25-35€):
Yes, Arragonis, they have Fabia covers
For other models: There are also lots of companies that make plastic grille replacements. You can contact some of them and ask if they would make a "full" version, without the holes, or with a sliding partial cover.
Wheel covers: Contact any company that makes plastic hubcaps and ask if they would make full, flat ones for you. You'd have to order at least 100-500 to make it worth while, in each size from 13" to 16". Estimated cost: 10-30€ for a set of 4.
I once sent a few e-mails, but noone answered
If you have more luck, then I'll be the first to order them from you
For alloy wheels you may try to attach them to the screws instead of the rims, like the mini-caps on Skoda Felicia wheels.