Okay, let's go with a personal mana store. Once you use that up, you can only recharge by drawing mana from your environment.
Priests can cast a simple spell that gets their deity's attention, then receive mana from him. Major temples have shrines that are magically linked to their deity so that some of the mana that they produce spills out here.
All mana comes from sources referred to as Fonts. Fonts can be absorbed by mortals, who then become gods (or at least demi-gods). More often, though, they're absorbed by other gods, who add them to their own power. If a god dies, their mana font is freed, so lesser gods have to worry about being killed by more powerful ones for their mana font.
Recharging mana from the environment requires a skill check. It might even use its own Mana Gathering skill. Priests wouldn't necessarily need that skill, but would depend on their deity for mana.
The difficulty of recharging is based upon the amount of mana in the area. If there aren't any fonts in a realm, then it can run out of mana completely, making it practically impossible to draw any up. I might use a rule like every X points above the base you get an extra mana point... or perhaps you divide your final Mana Gathering roll by X to see how many points you get... so in a mana-poor area you'd always get just one. But I need some sort of rules for doing it that don't require a dozen rolls to recharge.
Your maximum personal mana rating could be something like 10+DRV, or your highest mage skill + DRV. Or even your highest mage skill squared, although I think most spells are going to cost too few MP for that to really work. But I kind of like the idea of making it dependent upon your highest spell-casting skill.
Do I want to encourage specialization? Currently, the skill-point cost system will discourage excessive specialization by making it prohibitively more and more expensive as it goes up. I could further reward generalist mages by using the sum of all of your mage skills to determine your mana limit, rather than just the highest. Or it could just be based on your stats and thus not have any basis on your skills. Or it could be based on your Mana Gathering skill... there might not even be an upper limit as such, it's just whatever you rolled... given sufficient time, it would always be based on 20+DRV+Mana Gathering.
Hm. After writing up all of the options, I'm kind of inclined towards making your mana limit based on the Mana Gathering skill. It's a skill, so you can increase it fairly rapidly. And if we normally use just the flat value for your limit, we can let you recharge to full given enough time. Your new MP rating might be something like DRV+Mana Gathering+2d10 minus 20 (if it's lower than your current rating, there's no effect). If you take lots of time (hours), it's assumed that you roll a 20. If you want to roll, you can do it in mere minutes, but since there's only a 1-in-100 chance of getting a 20, you'll almost certainly get fewer MP. You can also super-rush it to a single round, but the roll is at -5. Powerful mages can get effectively infinite mana that way, though, by rolling DRV+Mana Gathering+2d10, subtracting 25 and still getting a decently high number.
Let's see, if you wanted to get an average result of 5 mp despite the -5, you'd need... DRV+Mana Gathering of... 30-11=19+. Ouch. A god could do it, of course, since they get to add twice their Mana rating to this roll. We could also drop the base difficulty by 5, then you'd default to DRV+MG+5 as your max instead of DRV+MG. If you gathered mana in a region where there was a bonus to the roll, you could actually store still more MP than normal, so mages could actually supercharge themselves if they had access to a powerful mana source (assuming I allow it; it would be easy to rationalize saying that you can't hold that much mana so it bleeds off rapidly, if I wanted to).