Go for the low hanging fruit first.
First thing I would do is email all of your past clients. And let them know if they are wanting a commission as a xmas present they need to get back to you before November 30th (or whatever date you choose). If you want, you could do an ‘Xmas special’ have a discount for the first X people. Offer this to existing clients first (saying something like, ‘as a past customer I wanted to offer this to you first’)
I would then also post the same thing a day or two after all over you social media.
If you don’t have a budget, find instagram hashtags that let people know you are an artist for hire: eg #artistforhire #portraitcommission – I made those up, so make sure you do your research.
There are also a couple of reddit forums that you can post on, eg r/artstore
You could also participate on r/redditgetsdrawn - When you post up your submission you can state you also do commissions. I've seen people get work that way.
Here's the thing, if you have already been posting up stuff on your social media etc, they are already through the first three steps of AIDA. This will get them to take action.
That’s just a quick brainstorm. I’ll post up any more ideas if they come up. Let me know how you go.