I had several from HF that I dropped off, I don't know if they got used or not though. The HF metal clip ones are junk and not worth your time even if you get them all for free. The black and gray plastic ones are OK and you can get them in a 3 pack for $3. I also recommend the craftsman ones though, that's what I ended up using at my place. They're manufactured by Ullman, and here's the cheapest price I found on them:
They're nice because the rails are all the exact same so you can mix and match the clips as you see fit. The ones I linked to don't have the plastic handles so I just put a red zip tie on each rail for SAE and a blue one for Metric. If you want the handles, Sears in-store is the best deal I saw on those. The spring clips do wear out after heavy use but replacements are cheap. I picked up some additional rail clips on Amazon. They sent out way more 3/8" clips than I actually ordered (or could ever use) so if you folks go this route and need additional 3/8" Ullman/Craftsman clips, let me know.