Considering this is going to be a long term installation, I would nail or screw clear fir 1X4 or 1X6 lumber onto the inside of the opening you have now, and stain and varnish (if you want the wood to show).
You'll have a gap behind the 1X4's or 1X6's, but you can use shims in that gap to ensure the top lumber is horizontal and the sides are vertical and the correct distance apart. Then, use whatever wood molding you want tacked to the edges of the 1X material to cover those gaps from showing. A folding door is light and fir 1X4s or 1X6's are plenty strong enough to support that kind of a door.
If you prefer to prime and paint, you can opt for a lesser grade of lumber, but I'd stick with fir or pine because they're both a lot stronger woods than spruce.
Where I live, the grades of fir lumber go as follows:
1. Clear - no knots or defects at all. This wood is only used where it will be seen, such as for wood moldings or furniture.
2. Export Grade - This is the highest grade of wood that you can get without paying a lot extra for Clear. We sell it to help support our economy, and it has to be good to compete successfully with lumber available from other countries.
3. Select Structural - has fewer defects than construction grade lumber
4. Construction Grade
Typically, you can buy Clear and Construction Grade in any major city. However, you may have to phone around to get Select Structural, and typically Export Grade will only be used for export and won't be available within Canada.
Then, it's just a matter of cutting the wood down to size by either ripping it on a table saw or planing it to the exact size. Probably your best bet would be to rip it to 1/16 to 1/8 inch oversize, and then plane it down to the final size to get the smoothest finish. Any of the hardware stores/lumber yards that cater to the woodworkers in your area will have a plane and can do this for you. And, every lumber yard will stock or will be able to order clear fir for you.