A couple of years ago I ordered custom steel/aluminium folded enclosures . I simply picked 20 firms from Google (from Germany because I thought it would make the overall discussion and transaction easier), mailed them a Sketchup design and asked for a quote for a couple of different quantities. Prices would differ by a factor of up to 5 between different companies, so getting more quotes definitely paid off. In this case I only sent them a drawing and they would do the model in Solidworks because they used it to generate the production data, so that was part of the price. Almost all companies were willing to do small quantities or even single pieces, you just have to be prepared for the price per unit to skyrocket due to initial setup cost etc.