I converted many bicycles to single-speed ones.

I would not get a bike shop to do it because labour would be a lot.

I had parts on hand and thus the cost to me was either zero or very low.

Here's an example of one I did by using a single-speed freewheel and an existing double crankset with one chainring removed and the mounting bolts shortened.




It can be a fun and extremely satisfying project. I tell people to check their local bicycle co-op for donor parts if they don't have them on hand.

Good luck and cheers