Read this informative article by open hardware advocate Bunnie Huang - on how to make a proper Bill of Materials - http://www.bunniestudios.com/blog/?page_id=2812
See example 
General Protocol - OSE International
Search the web for suppliers serving the area.of interest. For the seminal workshop, this is the Maysville, MO area in the USA, zip code 64469.
Suppliers can include local Hardware stores, such as menards, home depot, and Lowes as the main ones in the area. Mokan Fastener is the most favorable local industrial supply of bolts. Metal by the Foot is a local contractor supploer of all steel sections, up to 3" diameter rod or larger. There will be other industrial suppliers, McMaster-Carr being the most favorable, followed by Grainger and Misumi that specializes in cut to length aluminum extrusions. Locally there is the Sweiger Shop as a heavy metal fabricator. Then there are various global online stores. Amazon and ebay are common sources. And ultimately, there is Alibaba and China, which has many OEM sources, where one needs to email and get a quote. Sometimes these OEMs will have a formal RFQ forms, or less formal email. Relevant considerations there are lead time needed, as well as Minimum Order Quantity (MOQ).
In order to promote open-sourcing of the supply chain, we can collaborate with other open source enterprises and request that they share their supply chain information with the world. Our allies may include entities like Lulzbot, Prusa Research, Folgertech 3D Printers, Andrew Graham, who is currently developing a kit, Chris Palmer who has sold kits before, Adrian Bowyer who started the open source 3D printer movement, Ultimaker, and Zach Hoeken, one of the original creators of Makerbots - along with OSHWA, Sparkfun, Adafruit, and others.
Further we can also invite those entities who typically do not share their IP, such as Stratasys and other proprietary vendors, OEMs, and suppliers.
The critical aspect of supply chain management is to perform an exhaustive search of sources. This process should be adapted to various locations around the world - such as a producer in the USA may naturally have a different metal supplier than an Indian producer. Thus, this template should be localized for any OSE branch. An existing Template may be used as the starting point.
The IP value of the open source supply chain is that the supply chain listing should distinguish between reliable and unreliable sources - on grounds of product quality, shipping, service, or other factors. The difference between a good supplier and bad support can mean heavy losses, or even going out of business. Supply chain risk is perhaps the most u controllable risk in distributed production, for which reason OSE's ultimate approach to solve this issue at the core is Technological Recursion and Import Substitution via relocalized production and supply chain creation within minimum distance practical, which should be no more than one hour travel distance. This facilitates not only a Robust supply chain, but also rapid innovation cycles within OSE's framework as a learning organization striving for continuous improvement.
The template below shows space for 5 suppliers, which should reflect convergence to the best supplier (in cost, time, quality, reliability), especially in that supplier prices can be compared to OEM prices. Research from trusted references or product revuews can help determine the actual product quality.
The critical skill in creating a good BOM is determined when the search has been exhaustive, and the best decision has been made. To make this decision, references, reviews, or recurrence of a similar price for apparently the same features - are the best guide. Since the supply chain can change dynamically, reliability and start ility is important. The first choice of verifying suppliers would be word of mouth or referral from other customers, especially trusted customers.
Just like we create sourcing for the original OSE International location - sourcing should be locally adapted for most favorable sources for build locations other than Missouri, USA. Thus, there should be a whole set of BOM spreadsheets available - for local adaptation.
Specific Sourcing - 3D Printing
Tobben's previous preferred kit-supplier, reprappro.com, closed its sales on Jan 15 2016. They left a list of alternative suppliers on their home page including:
- The RepRap.org Website
- Trapletshop (Fisher kit)
- RS Components
- Mouser (Europe)