Add a warding Summoner Spell.
Have that Spell be upgraded as you purchase/upgrade your coin/relic/spellthief's.
Consider having coin/relic/spellthief's generate XP in some way if you have the warding Summoner Spell. The mechanism for generating the XP could be different for each item.
Clearly something is wrong with the support role, since very few people want to play it. I think part of the problem is that purchasing sightstone is not fun. Not purchasing sightstone is asking to be flamed and blind, which is even less fun. Let's just put it on a spell and let our support players start playing the item game 800g sooner.
"Everyone would take a warding spell" - If you gate the power of the spell behind upgrading one of the support items, you can prevent other laners from taking it.
"warding needs to come at a price" - Losing exh/ignite is a hefty cost. The stats/cost of the support item upgrades could be adjusted as well.