I don't know of any other sources but have made them myself.

Use hardware cloth (that's what the wire fencing is called), you'll need gloves, and something decent to cut the wire with. The shape you cut depends on the cage you want (obviously). I've dealt mostly with voles, so it was enough for me to cut a 6 inch tall strip and use it to line the sides of the planting hole. As long as the top of the fence was slightly exposed, the voles never made the effort to come up and step over.... they kept strictly to the top 2-5 inches of soil or mulch and only went deeper if they utilized a mole tunnel.

Others may have more suggestions, but for squirrels I think a top cover is most important, for gophers and chipmunks you'll need a completely enclosed cage.

When cutting the wire, cut through the centers of the squares. that way the ends can be used to mesh into the connecting piece without gaps.

My final solution was to move to an area where voles and sqirrels are less common. It's so nice to be able to grow a crocus or tulip without making a 2 hour project out of it! LOL

I also made my own bulb cages and put them around circular pots (that contain tulip bulbs) using wire fencing and tying it together with something similar to twisty-ties. I put a roof on top of the widest pot and bent the walls together to seal up the top in the smaller pots. The stuff is available at hardware stores. Yes, gloves are necessary because the fencing is sharp, but it's so tight that a squirrel couldn't get in if it tried.

There is two materials commonly used for making your own cages. If you only need them to last for a season or two, some soils quickly dissolve the wire, you can use aviary wire. Looks like chicken wire with smaller openings. For longer lasting cages half inch hardware cloth is recommended. The aviary wire is light weight and can be bent around with your fingers. The hardware cloth is better fastened with "hog rings". Buy the hog rings and the hog ring pliers at the hardware store where you buy the hardware cloth. After you have made the first one, take it apart and lay it out flat, using it as a pattern to cut the rest of the wire. Al