If you have a Pull-A-Part type place near by, you can salvage relays and sockets from newer cars. The one around here treats them almost as free stuff. A pile of relays and sockets will get counted as one misc $3 part. Your results may vary. One advantage of automotive relays is they usually include either a diode or resistor to suppress the inductive spike of the relay coil. I would still use the added 1n400x diode just to be sure. Hard to have too much spike and noise filtration. Vehicles tend to have lots of electrical noise wondering around. Something about 200+ amps of starter current and 50kv ignition sparks.
Another source for electronics stuff:
https://www.jameco.com
You should be able to get a 30a solid state relay.