Since I haven't seen any mention of this item & my LBS's don't carry it, I figured I'd post this in case someone else is in the same predicament. One of my bikes has a threadless headset that is too short for my tastes. I can ride it, but long distances become uncomfortable.

I was looking at getting a new stem with a higher rise, in hopes of making the bike a little more comfortable, when I stumbled across the Delta Threadless Stem Extender. It allows you to raise your handlebars up to 3"). I got one, installed it (took literally 5 minutes since my brake/ gear cables were all long enough) & my bike is a thousand times more comfortable. I'm not super great with a wrench & wasn't looking forward to replacing the whole headset (or whatever the piece that the stem mounts to is called), so I haven't been riding this particular bike as much as I would like. Best $30 I spent all year (Obligatory heads up-I have no affiliation with this product/company other than as a satisfied user of this particular piece of gear)

It would have likely been more than just the headset if you opted to to upgrade the entire front end. The Delta adapter goes from a 1" threaded to a 1.125" threadless. While you could have gone the route of a taller stem like some of the offerings from Nitto, they do require that you unwrap the bars and remove the brake levers because the stem doesn't have a removeable face plate. You could have swapped headsets to a 1" threadless, but then you need a new fork also.
I've got an adjustable 1" quill stem from Zoom, and it does use a 26mm removeable face clamp, but it suffers the same issue as the tall Nitto stems... When really cranking on the bars, it feels noodly. I think the use of the adapter and a threadless stem, while a less elegant looking solution, is certainly more solid feeling.
Got any pictures?

Sorry, it is actually called the Delta Threadless Stem Extender. After reading your reply, I realized that there are two separate items, once called the adapter & one called the extender. The extender (which I have) only lengthens the piece that the stem attaches onto. I'll go grab some pics & post them.

stem raisers go in place of the stem, on the steerer, then provide a location
to clamp your threadless stem onto..

there are a couple other types.. different clamp to the stem mech, same function.

Here are a couple of pictures. If they don't show up, I did this completely wrong.

I've had one of those on a single speed I bought at a garage sale that was just a little too small for me. Been using it for about 1 1/2 years and no problems. When I orignally put it on I just tightened the bolts to the same (estimated) torque as the bar stem was originally tightened. Recheck it occasionally in the first few weeks of use to make sure its still secure. I wouldn't use it on an MTB that'll be jumping curbs or used on dirt tracks, but it works great for road riding/commuting. If you don't like it you might be able to put some BMX-style riser bars on that Marin.