Assuming your Spiratone lens has a Nikon mount, there isn't an adapter for it. It either fits or it doesn't. If it doesn't, it's possible to have someone machine the back to make it fit, but given that the value of the lens is only $10-20, I wouldn't mess with it.
Your updated note mentions a Miranda screw mount. This wasn't common on Mirandas (most of which used bayonet mounts), but it's the same mount used by Pentax and is often called the M42 mount. You can find cheap adapters on eBay that allow for infinity focus by including some additional optical elements, but I have to tell you, I wouldn't do it. The lens will be cumbersome to use since it won't have an automatic diaphragm, making focusing and metering awkward. The optical quality will also be degraded, and it wasn't a stellar lens to start with. It's really not worth it. If you really want to try it, just search on "M42 Nikon adapter infinity" on eBay, and you'll find some for about $8-10.
The Miranda screw mount is 44mm so the Pentax M42 may not fit. However I have seen some references to m42 mounts and 44mm mounts. I can't think of one reason to try and use this lens on a Nikon. It's performance probably isn't very good. I would forget about it.
Your photos are quite good. I especially like the seagull and the rose. I think it worked out pretty well for you.
It's been quite a few years since I bought anything from Spiratone. That was when they were still in business. In those days, the mid 60's, I was using a Pentax Spotmatic. If my memory serves me right the hot item at the time was a 400mm lens. I tried one but found it wasn't very good. I also bought a Spiralight flash which I still have somewhere and I think it still works. Some of their products were fairly good for the price and some not so good. But they served a purpose to the not so rich (poor) amateurs. That described me pretty well in those days.