Rock tumblers and pea gravel don't cost that much. In theory, anyone who wants a stonewashed finish should be able to get it fairly easily. Might have to do a "pre-wash" with sand to get the spaces between the heatsink fins, though.
Unfortunately those small inexpensive off the shelf rotary rock tumblers made for polishing rocks will not work, well maybe for a Lummi Wee sized light. They just don't have the capacity to provide enough tumbling action for stone washing. Tumbling for a long period of time with one of those will just give you a beadblast appearance as will using smaller media. For stone washing, don't expect the inside of grooves, fins, & corners to get stone washed.
With the right equipment & media, tumbling for stone washing isn't done for a long period of time to get the effect. Tumbling for long periods of time is just deburring which is not what you want for stone washing.
Stone washing is anything but easy. It's not as simple as tossing something in the tumbler, flip the switch, & have a beer. Stone washing will not hide anything, in fact it will make machining marks, scratches, dings more noticeable. Which means you have to put a nice finish on it before tumbling. Some people even bead blast after finishing before tumbling. But once stone washed it will help hide any new scratches & on steel will be pretty resistant to surface rust. On a flashlight you would also have to fully disassemble the light in most cases & cover/protect threads, knurling, etc.
Stone washing equipment is not cheap & it can be as time consuming as polishing. Probably one of the reasons why you don't see it much on flashlights, that & the look is an "aquired taste". I rather anodize something in Ti (simple patterns) anyday over stonewashing, it's so much more less time consuming & most people like the pretty colors better.
But an inexpensive method for stone washing is the strong arm method. Get a container large enough to hold the pieces & media & throw in some of your "special" rocks from the yard, add some soapy water, then shake until your arms fall off or until you get the look you want. Hopefully you'll achieve the look you want before your arms fall off.