Somewhere between scarce and rare in my neck of the woods. I would guess I've seen maybe four in the past year that were for sale, and only a couple of them in excellent condition and one of them I brought home with me.
Here's a link to one that sold just a few days ago. http://www.gunbroker.com/Auction/ViewItem.asp?Item=47513859
The Troopers were built on the same size frames as the OP, OMM, and the Python. they were available in 22, 38, and 357 in 4" and 6", but all the 22's I've ever seen had 4 inch barrels. I don't think the 22's were offered in 6 inch but in those days Colt would give the customer just about anything on special order, so I wouldn't bet my life there are no 6 inch 22 Troopers in existance just because I've never seen one.
If you search within the revolver forum, you will find a fair amount of information of the original Troopers. I would rate their availability as "Scarce".
Folks generally shot the Troopers when they bought them, so you find a lot that have been shot in the 95%+ condition. As a friend used to say, "good honest guns". They are a fine heavy barreled 22 revolver.
Summary specific to the 22 Trooper:
The Trooper revolver was essentially an Officers Model Match revolver with a 4-inch barrel. Some say that the action was not was well honed as the Officers Model revolvers, but I don¬ít know.
Production appears to have been continuous from (late 1952, Serven), 1953 through 1962 for the .22 LR Trooper. Selected (available references) all include the .22 Trooper model within this time period. The model was replaced by the Trooper Mk III in .357 Magnum/.38 Special in 1969, but not with a 22 until 1979 when the Trooper Mk III was introduced in .22 caliber. So between 1962 and 1979, no Trooper model revolver was available by Colt in .22-caliber.
I have to agree with the earlier posters stating that the .22 Troopers are scarce, edging towards very scarce. I have seen few nice ones for sale in the past 5 years or so, a reflection perhaps that post-war Colt DA revolvers are increasingly sought after.
Colt probably did NOT need to offer a .22 OM Trooper in a 6" barrel,as they already had the Officers Model Specials. Identical gun,except if someone specified the non target hammer and service stocks on a .22 Trooper during its time frame of production.
My old model .22 trooper is just as accurate as my 1932 vintage Officers Model .22.
I can no longer find it nor can I remember the source, but I used to have information that Colt only made about 2200 old model Troopers in .22LR.
The .22LR version was only available in a 4" barrel since anyone wanting a 6" version could buy the Officer's Model Match.
Of the .22 Troopers I've been inside of, they were as well fitted as the Officer's Model Match, but I don't think they were quite as well tuned as the Officer's Model Match.
Later Models seemed to be about the same, and this was a case of the Officer's Model Match NOT being tuned to the old standards.
The Trooper .22 was intended to be a "trainer" version of the center fire models.
In terms of rarity, models fitted with the FACTORY ORIGINAL Target hammer and/or grips are the most rare.
The vast majority of them had Service grips and hammer.
Since Colt offered these as options, you'll find Troopers of all calibers fitted with any combination of Service hammer and grips or Target versions.
In other words, Troopers were shipped with Target hammer and Service grips, Service hammer and Target grips, etc.
On thing to be aware of, it's not at all uncommon to find Colt's in which previous owners robbed Target parts, or ADDED Target parts.
My 1954 Trooper .22 was originally fitted with Target hammer and grips, but years ago someone robbed the fully checkered Target grips, but left the Target hammer.
Later someone added a set of Second type Target grips.
I'd very much like to buy a set of First type fully checkered grips to restore it, but at current prices, I simply can't afford it.
Years ago, I did change out Service hammers on Colt revolvers on customer request, and people also did this on their own when selling a gun and wanting the Target parts for another gun.
Strangely, I once had a request to install a Service hammer on a Python.