I am using 2-4 foot limestone boulders as rip rap in a nearby canal. I'm trying to avoid buying EXPENSIVE mining/lapidary equipment. It is very SLOW work however to cut off the sides of the boulders to prevent them from rolling down the embankment. I have been using 9 inch grinders with diamond blades. Turns out limestone is kinda hard. It is so porous, I thought it would be faster cutting. I have used feather and wedge, but with mixed results on this very holey limestone. Are there any better ways?
Use concrete to stop the rocks from rolling, you can get white cement (for a small extra cost over the price of grey cement) and if you use it with white aggregate (e.g. quartz or marble chips) it will be basically the same colour as limestone,.
Get a larger drill or electric jackhammer. if you don't like the price hire one for a few hours and see if it does the trick.
A grinder is the wrong tool, you really need a wet saw to cut rock, the water will reduce the amount of airborne dust, and save quite a lot of wear on the expensive blade.
If you take some rock samples to the tool hire place they may be able to offer advice and may even let you try their tools out for free.
Find a source for smaller rock that will not have a tendency to roll. In a very similar situation I used defective bricks. There was a brick factory in the area and I found they sold defective brick very cheap, essentially fill. The full and half bricks were very stable in a very fast river bank, faster than your flow. In a heavy rain my stream rose over 12 feet, sort of a ravine with flat shorelines. So a half brick size gives a general idea what size you need. My soil was sand so very susceptible to erosion. I did add many cuttings of water tolerant brush like redtwig dogwood, but the brick was essential .
Do you get cold winters?
A traditional mining technique in the UK was to expose the limestone bed, cut vertical holes and grooves in selected places (with hand tools!), fill them with water, and wait for a winter or two to do the rest. The water froze, expanded, and cracked the rock. With a quarryman's skill, it cracked into rough cuboids that could be built into walls without further shaping being necessary.
Anyway, experimentally, you could try drilling a few vertical holes into some boulders, filling them with water, and wait to see what winter does to them. Experiment with hole diameter and depth and spacing.
Obviously if you do not get cold winters, this idea won't fly.
The other answer which probably isn't what you want, is to use gabions. These are enclosures made of tough approximately 2 inch square mesh) , something like two feet cubed. You fill them with crushed rock. By the time the wire mesh rusts, weathering, dust, and vegetation has bound the crushed rock together. It's a common technique (in the UK at least) for reinforcing unstable river banks or embankments.