Caveat emptor

 Putting together a S100 backplane at the very minimum requires hard to find edge card connectors. 0.125" pitch is no longer in common usage. The N8VEM V4 backplane uses a LM4250 op amp which is obsolete. Zilog still produces the Z80, but that's the only CPU that I know still in production. Legacy boards have "who knows what" on them. S100 computers will be an exercise in second market, small electronic shops, eBay and New Old Stock (NOS).

Here is my experiences chasing down three parts, the edge card connector, the LM4250 and a SII164CTG64.

 There are a bunch of edge card connectors that could be used. One still in production is the EDAC 346-100-520-802. If you are willing to spend about $14 a connector (including tax and shipping) you can order them from any of the usual suspects. eBay can be crazy! They could be cheap and I've seen "vintage" parts going for over $100 / connector. I suppose someone may think they are collectible. The best place if they are in stock is a reputable electronic components reseller/liquidator. In general they are more interested in moving stock and "making it up in volume".  

I bought three connectors at an ok price off of eBay, they arrived in three days. I picked up another two from Electronic Surplus, but I'm not as happy with the connectors (No fault with Electronic Surplus, one is just a wire wrap connector and the other doesn't have mounting holes). Here's where it gets interesting. I   found a site that had a good price with free shipping, they had PayPal as an option so I ordered a bunch. A week later the site is GONE. The good news is I tried AliExpress and Alibaba next. AliExpress was a bust, too old and too obscure. Alibaba was amazing though. I found at least THREE manufacturers that are willing to deal with individuals. I went with a supplier that had no Minimum Order Quantity (MOQ), had a good price on each connector and shipping. If I want to go with a MOQ of 100, I can get the connectors for $1.20/piece + shipping😄 (I have an idea) And the people I was interacting with were very patient with me!

 The LM4250 is an obsolete part no longer manufactured by TI. Rochester Electronics makes one under license, but I couldn't find anywhere to order them. My next thought was to just replace it with another op amp. While not strictly interchangeable, op amps largely work the same and this one is used to provide a reference voltage for an "odd ball" voltage regulator. One lively conversation later and replacing it is a distance memory. (My knowledge of op amps is significantly expanded, which is a bonus). I ordered a lot of 5 from Futurlec and another lot from AliExpress. Futurlec has been around for a while and has a mixed reputation. The impression is that they do ship out product, eventually; it could take a lot longer than you would normally expect. AliExpress I've had good luck with (so far), but they ship things on the cheap. Expect two to three weeks for shipping. At least they offer a great deal of visibility into their shipping process.

 I ordered a run of 3bpp Black Mesa HDMI pMod pcbs. They are pretty simple, but require a SII164CTG64 and they are out of production. I ordered one off of eBay and it showed up in the usual couple of weeks. I decided to order a few more for backup and to finish up the other boards I had on hand. A month later, no shipping notification, no communication from the seller. :(

Take aways:

Before you drop any serious money on a product from a site that's new to you, do at least a cursory search online. My debacle with the scam site and the card edge connectors could have been avoided this way.

Use intermediate sites. Amazon, AliExpress and eBay offer buyer protection. I've used them before, generally it's pretty painless.

Use PayPal, they also offer buyer protection. It's there, also pretty painless. They usually offer a percentage up front or 100% if you ask them to investigate.

Finally, your bank, credit union or credit card company will let you dispute charges. Generally they try to side with their customers but YMMV.