Can't beat free, I think I have about $ 35 into my Montgomery Ward 610 and I'm still looking for a set of felling spikes and hardware.
A while back I made felling dogs for my old 610 out of some 3/16" steel. If you have the owners manual, there is a side view of the saw with dogs on it. I photocopied the picture and enlarged it until the mounting holes for the dogs were the right distance apart. Basically just trial and error until I had the right scale. Then just cut out the shape of the dogs on paper and traced the shape to the 3/16" steel. Then I cut it the steel out and shaped it with an angle grinder. Drilled the two mounting holes and bolted it right on. They have worked for the last 9 or so years and the cost was $0. I don't use that saw much anymore, but it is a good backup. I don't have a way to scan in the template I used to make the dog, but you could always freehand sketch one if needed. If you have no luck finding or coming up with anything, pm me and I could mail you a copy of the template I have.