Hi,
Probably not very easy to find someone that has done this already. I've done similar jobs before however, migrating from a platform to another. What needs to be done is a custom written script that fetches the data from Nopcommerce into a big set, organizes it under the shape Magento is expecting it and inserting it into the Magento database.
This is not a very easy job. Also consider that migrating might not be at hand if the products from Nopcommerce don't match the Magento ones.
The other problem is the password encryption. The algorythm used to in one will not match in the other, so you'll need to reset the passwords for all your users.
I can help you in this matter. But the budget and timeframe is an unknown and strictly depends on what info you're looking to migrate.
Thanks much,
Cosmin