Holsters are easy to find for just about any gun...Unless of course you are left handed like i am.
If you are left handed then plan on having to order one because you probably wont find one in a store.
Any particular gun you were asking about?
You probably should have mentioned in your first post the guns you needed holsters for rather than asking what was hard to find. I mean any gun can be hard to fit into a holster when modifications have been made or the gun is a new model and somewhat different than anything else. Moreover, it's hard to find a holster if you are very specific in your needs. For example, if you had a Colt 1873 Single Action Army (Peacemaker) 8" barrel variation, and you wanted Austin engraved with a hammer loop and matching belt; you might have a harder time than if you just wanted any western looking holster for that model with a 7.5" barrel.
I like Vedder Holsters in Mt. Dora, Florida. Unfortunately, they don't make a holster for that Ruger MKIV. I do notice one of our official vendors at the Handgun Forum is Tandemkross and they do indeed make holsters for your Ruger.
I bought a Blackhawk omnivore last week for my p226 that now has a TLR1 hl lite on it. On the box it says 150 semi auto pistols will fit in them, no matter the bbl lenth too. It's not for conceal carry but will be fine for duty or woods carry!