I suppose it's just a matter of calculating your total costs. If the board you want doesn't come with an IO shield, then factor the cost of one into the true total cost of your motherboard. If it's too much, then just be patient and wait for a seller that is including the IO shield with the board.
I once knowingly bought a motherboard without an IO shield, thinking that I had one already. Well, the IO shield I already had didn't fit it perfectly. I used a dremel and some metal tape to make it fit. I've also used this stuff to fill in the empty holes on cases that have knock-out panels that are missing. It works really well. I use several layers on both sides. It's not as good as having the original metal in-place, but it comes close.
If you want to do an extra nice job, I'm sure you could also buy some thin vinyl or plastic sheets and make something pretty easily. It would help if you have another IO shield you could use as a template. I know I've seen plastic binders and folders that could be easily cut to make an IO shield. Be creative.
You could also consider getting friendly with your local electronics recyclers or thrift shops. Be nice to them and they might let you pull a part from something they've taken in.
I don't own a 3D printer, nor have I ever used one, but have you considered that option? If you can't download the plans to 3D-print the IO shield, maybe it isn't too hard to make one yourself based on measurements? My local library has a 3D printer as well as folks who know how to design things. It's something to consider...