Depends on whether you want speed or convenience. HKS is fine for convenience, but requires two motions (insert / twist knob) to activate, and as pointed out, relies on gravity to get the cartridges into the chambers.
Safariland makes three different speed loaders - the Comp I, II & III. The I & II also rely on gravity, but only take a single motion (insertion) to release the cartridges (the release bears against the extractor star). The II as a larger knob on the top making it easier to grab from whatever carrier you use. The III also is activated by insertion, but uses a spring to push the cartridges into the chambers - much more positive and faster. A favorite of competition shooters. Has a large top knob for fast easy grasp, but makes not suitable for concealed / duty carry.
Jet loaders are similar to the Comp III's in regard to being spring loaded. There are other brands out there, but the HKS and Safariland are the most common and easiest to find, and both are quality products. I think that you will find that any manufacturer of speed loaders will have offerings for K-frame .38's.
To expand on grwr's post #4 - S&W Target grips came three ways over the years. No relief cut at all (the earliest). Extractor relief, also known as "Football" grips, and speed loader relief grips on which the top of the left grip panel is scalloped to the metal to provide clearance for the speed loader body. These were not produced concurrently, but on a timeline as listed above. There are other S&W grip variations, but these are the ones most commonly encountered in target grips. Most aftermarket grips provide speed loader clearance.
Extractor relief grips (Football)
Speed loader relief
Adios,
Pizza Bob