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Get some hockey pucks, drill 5/8" holes in the exact center of them, and put them over the springs, but under the top hats. This will preload the springs, but also takes away about half an inch of travel from the spring itself, which doesn't matter on stock struts.

You can also put a set of front springs on the rear struts. The fronts are always stiffer because of the extra weight.

Basically, find a heavier car with coil-over strut design, and use them.

You can find self-install coilover kits on eBay for under $100. They have heavier springs (usually ~350-400lb/in), and come with sleeves that allow you to adjust the ride height. If you want to keep stock height, wind them all the way up. You may also need to add spacers, since the springs in those kits are generally 2-3" shorter. If you add a spacer, you can use the hockey puck trick, or you can cut a piece of pipe that will fit over the strut body and still support the sleeve.

If you're an avid junkyard hunter, find some CRV springs. They're close to the right length, and only slightly heavier than the 4th gen Civic ones. You'll need a spring compressor.

If the yard makes you buy the whole strut assembly (they usually will, for liability purposes), you can disassemble the springs at home by stepping on the spring itself, then putting an air impact driver on the top hat nut and spinning it off. The strut will push back away from the spring up to 10 feet, so pay mind to what's around you. To install the springs on your stock struts, you'll either need a spring compressor or a floor jack with a wide saddle and a couple pieces of rope/cable. The latter method, you place the spring in the jack saddle, then jack it against a solid object with the rope strung through the inside. Once it's compressed, tie the rope tightly and release the pressure slowly from the jack. You can then install them, tighten the top hats down, install the struts on the car, put a jack under the lower control arm and seat the spring, then cut the rope out and slowly lower the jack.