I have seen them loose in lots on occasion, but they are easier to find mib as a lot of people bought those sets to keep in package. Loose I'm guessing you wouldn't pay much. Under $4.00.
I hope to find some re-issues loose for nieces to play with when they visit, as I'm worried about chemicals present in the vintage versions.
Oh, old starshine is much easier to find. 83' ponies were manufactured on a massive scale, she was also re-released with the 84' rainbows as well. Vintage she is htf without glitter issues and discolouring.