Roll20's JSON for spells, as mirrored by the roll20.json spell file in 5etools data, can be found e.g. here. Simply change the name of the spell to view others (provided they are part of the SRD).
If you want to import from the main site instead of the script's data, there's a hidden option d20plus.debug.forceExternalRequests that causes the script to pull from the main site (or external URLs) no matter what if set to true. It is false by default. This option can be set both by altering the script and by typing d20plus.debug.forceExternalRequests = true in console.