David Barnett
I know I promised at least two persons a couple weeks back I'd post where to buy cast iron for making laps and cheap diamond. Rather than go through my email backlog, let me just post it here before I put it off any longer.

If you're not going to look for scrap cast iron, new will cost you more, but it's handier for some folks to just order it and get going.

I suggest if you want just one lap big enough to do most sharpening work, a 6" diameter x 1" thick round will work nicely for freehand honing. For those who simply must use a guide, a rectangular lap is more what you'd want, but the following source only carries 24" long plate. This means you'd end up with three 8" x 4" laps, or two very nice 12" x 4" laps. Problem is, you're out more money initially, may end up with one or two extra laps, and if you should be unhappy with diamond on iron (you won't be), you'll have spent more than you probably intended to try out the process.

A straddle might be to purchase an 8" diameter round and work with that, as most who use honing guides are accustomed to 8" long stones, so this shouldn't present a problem.

The price for 6" or 8" diameter rounds will run $23.81 and $37.58, respectively.

The catch? These aren't laps. Not yet. They come rough sawn to to 1" thickness from a 6" or 8" rod. You'll have to flatten them yourself or have them flattened. If you know a machinist or can locate a machine shop who will help you grind your iron blank flat, you're ready to go. If not, you can do it yourself with very simple hand tools and some time and effort. This is how I flattened mine and I found it entirely enjoyable. It's a good way to flatten iron plane soles, too. Really, you can do this.

The first thing you'll need is a large file to take down the highest spots and work below the saw marks. The edges of these rounds or rectangles will get sharp, so break the edges frequently with the file as you progress. You can also wear Kevlar gloves, which are cheap and do a good job of protecting you. After getting your blank roughly flat, checking with a straight edge or ruler, you'll move on to scraping.

For scraping, you'll need three things; a flat reference surface such as a cheap granite surface plate or slab of glass, a tube of thick oil paint (Prussian blue is traditional, Dykem is a machinist's brand name), and a machinist's scraper, which you'll learn more about later. By the way, if you want to scrape both sides you could have a double lap although I don't recommend it to start. Contamination can become an issue, so begin with one lapping surface and one diamond grit (14,000).

As I said in one of my email replies, I won't be teaching you to how to scrape but I will give you some sources that should do that.

Again, my suggestion is to go with a round blank for sharpening. I use 6" rounds myself. If you intend to flatten wide plane blades, you'll probably want to go with 8" round or rectangular, and ideally, 12" laps. But for a 6" round lap, if you order from these links to MSC, you'd need:

6" cast iron round at $23.81
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1986352&PMTERM=63379879

Size 8 machinist's scraper (13" overall) at $5.92
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1747076&PMTERM=05844154

and a tube of Dykem Hi-Spot Blue at $7.05
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1658978&PMTERM=00264101

Alternatively, you could substitute:

8" cast iron round at $37.58
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1986353&PMTERM=63379887

...or a 3/8" thick 4" wide by 24" long cast iron plate at $116.46
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1986373&PMTERM=63380083

...which would give you three 8" x 4" plates at $38.82 each, or two 12" x 4" plates at $58.23 each.

Diamond compound is expensive from MSC, a 5-gram syringe of 14,000-grit medium concentration at $18.35 each. To some, it may be more convenient to order everything from one supplier, so here's the link.
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=1745043&PMTERM=05652441

Cheaper is diamond from Daniel Lopacki, with any 5-gram syringe costing $5.95 each, in 360, 600, 1200, 3000, 14000, 50000 grits.
http://www.lopacki.com/polishes/

Kevlar gloves with rubber palms for secure grip such as these can be had for under $10.
http://www1.mscdirect.com/CGI/NNSRIT2?PMPXNO=20837399&PMTERM=84491208

Basic instruction.
http://www.schsm.org/SCRAPING.pdf
http://www.oocities.org/plybench/scrape_posts.html

Lastly, keep in mind shipping is apt to be non-trivial when cast iron is involved.

Hope this helps.