From the National Center for Home Food Preservation:
Making Jelly without Added Pectin
Making Jam without Added Pectin
- Use a mixture of 3/4 ripe and 1/4 under-ripe high-pectin fruits. Under-ripe or just barely ripe fruit contains the most pectin.
- Cook the fruit with cores and peels to add extra pectin (but do remove stems or pits). Put through a sieve before adding sugar and spices.
- Citrus peel contains lots of pectin, so consider adding some of it to your mixture.
Fruits low in pectin: apricots, blueberries, cherries, peaches, pears, raspberries, and strawberries. That's not saying you can't make jam/jelly/butter from these without added pectin. It just may be a little more difficult than, say, using apples. In fact, as an example, here's a pear butter recipe w/o pectin.
This depends on what you mean by a gelatin "substitute".
What you have to understand is that while most hydrocolloids have gelling and stabilizing properties, they are not simply interchangeable. You can't substitute one of them 1-for-1 where you need gelatin and expect everything to just work.
A great place to start would be the Hydrocolloid Recipe Collection which, despite its name, is almost more of a cookbook, because it has detailed information on the properties of each hydrocolloid.
Agar is actually a stronger gelling agent than gelatin in the sense of having to use less of it to get the same strength, but you need to use it properly. The most important property of agar is that unlike gelatin, which gives hydration at temperatures as low as 50° C, agar requires a temperature of 90° C. In other words, you need to heat the water all the way to a rapid boil before the agar will actually "activate". A light simmer is not enough.
The other notable property of agar is syneresis, which is the loss of moisture over time. Agar sets extremely fast compared to gelatin and above room temperature, but unless you combine it with a small amount of Locust bean gum, it will actually dry out. Otherwise, though, you can absolutely, definitely substitute agar-agar for gelatin if you actually get pure agar (I made the mistake of buying the "dessert agar" once, which is not the same thing) and hydrate/set it properly. In fact, the biggest concern with using agar as a substitute for gelatin is that you might end up with something too stiff, since gelatin produces a much softer gel.
Perhaps the closest hydrocolloid to gelatin in terms of its properties is iota type carrageenan. Here's a side-by-side comparison of the most important characteristics (this is all taken from the HRC):
Property | Gelatin | Carrageenan | Agar
------------------+-----------+-------------+---------
Thermoreversible | Yes | Yes | Yes
Strength | Soft | Soft | Hard
Elasticity | Elastic | Elastic | Brittle
Shear Thinning | No | Yes | No
Hydration | 50° C | 70° C | 90° C
Setting Temp. | 15° C | 40-70° C | 35-45° C
Setting Speed | Slow | Fast | Fast
Melting Temp. | 25-40° C | 45-80° C* | 80-90° C
Viscosity | Low | Medium | Low
Gelling Conc. | 0.6-1.7% | 1.0-1.5% | 0.2%
Syneresis | No | No | Yes
You should be able to see by this how much closer carrageenan is to gelatin; trouble is, it's difficult to find, and you have to get the right kind (the kappa type and other types have very different properties).
There's actually an even better type of carrageenan to use a gelatin substitute, if you can find it: It's called Ceambloom 3240 and it's specifically designed to be a gelatin replacement.
I'd like to also note for the record that the answer in your ochef link is not really appropriate for gelatin desserts that need to hold their shape. Xanthan gum is a fantastic and versatile hydrocolloid but (to the best of my knowledge) it does not "set" the way that gelatin, agar, or carrageenan do. It's more of a thickener/emulsifier/stabilizer, at its highest concentrations being used to produce foams (but not gels). It's often used to stabilize other gels/foams but I've never heard of it being used to create a gel on its own.
Guar gum is also largely just a thickener, that you could use a stabler replacement for corn starch or arrowroot, which is also mentioned in that answer. None of these are appropriate at all for gels (desserts), they are only useful as thickeners.
Best Answer
It's the refinement that's the real issue: anyone who's braised a big joint of meat knows that a couple of hours of low-temperature stewing will net you large amounts of gelatin. Hooves and antlers were the preferred media, but anything that's got a bunch of collagen will work. Talk to your butchers shop, and see if you can buy some bones.
Refinement was done the old fashioned way: by hand. You skim the top to remove the scum, you add egg whites to degrease and to clarify the "broth", and then you strain and strain and strain and strain.
And strain. And strain. And strain. The skimming/straining process is similar to rendering sugar from sugar cane, if you've ever done it (slightly less esoteric, because people still like molasses). They obviously don't use egg whites in rendering sugar.
It's a huge amount of work (and OMG it stinks...do it outdoors if possible), and the end result is probably not going to be on par with the stuff you buy from the store. Unless you're hankering to re-invent an ancient technology just for the fun of it, I'd just buy it.
Edit: I dug around, and there are a number of products for straining jelly that might make it easier, and they should be able to deal with the viscosity of the gelatin soup. Unfortunately they seem to be made for smaller batches, and that may not be helpful if you're doing a huge kettle.