For Halloween, Spencer Hastings donned a Mary Queen of Scots Costume. To dress like Spencer, look for Elizabethan Costumes. The only Mary Queen of Scots costume examples I see are pretty ugly or have to be custom made and are thusly tres expensive.
Now, about a Mary Queen of Scots Costume wig… This will be hard. I have to be honest! You’ll want to get a short red wig that has curly hair. And that could be a problem. Plus, to be Mary Queen of Scots, it really needs to be red and curly. And since that’s pretty impossible to find… I have a suggestion! Look for a red, curly wig that you like. Now, you could cut it, yes. Maybe Youtube has haircut tutorials… I don’t know. But what I’d suggest is using clips and bobby pins to clip the hair up to make it appear short. If it’s a very long wig, you can even try to do a hairstyle of the time period. Start by parting in the middle (if possible.)