Get or create a costume "Staff of the Planes"
I couldn't find such an item in the rules, but the rules for staves creation allow creating new types of staves. It will be extremely expensive, but you'll get an item which will allow you to use Greater Create Demiplane up to 10 times before needing to find a more powerful caster to recharge it for you (until you reach a level allowing you to cast 9th level spells, of course - then you could recharge it yourself).
Also, keep in mind that most of the unique benefits of Greater Create Demiplane over it's "standard" and "lesser" are something you may use sparingly - you can create a functioning demiplane using the "standard" and "lesser" versions, and use the "greater" variant only for adding a permanent portal or change the way time work there etc. (which, since the "greater" variant affects more space of a plane, you will need less castings to cover your entire demiplane)
If you really have enough money, I'd get 2 different staves:
- A "Staff of the Plains" which I'd need help recharging but grants access to Greater Create Demiplane.
- A "Minor Staff of the Plains" which doesn't have the Greater Create Demiplane power, but which I'd be able to charge on my own - this one will be used for most of the plane creation / modifications I'd need.
Thematically, I think the "Staff of the Plains" should have something along the following powers:
- Minor Create Demiplane (1 charge)
- Create Demiplane (2 charges, or 1 charge if you pay extra during creation)
- Greater Create Demiplane (3 charges, or 1 charge if you pay extra during creation)
Maybe also some helpful powers for a demiplane creator / keeper:
- Plane Shift (maybe limited to travel between a created demiplane and the Material, Astral and Ethereal planes)
- Permanency (limited to effects on the created demiplane)
- Some other niceties such as Tiny Hut or Magnificent Mansion etc.
If you want me to add the costs for such staves, or more information on how to create / charge them, please indicate that in the comments...
At any rate, this is something you'll need to work out the details with your DM - he may require you to research the creation of such an item and create it yourself, rather than simply allow you to commission such an item from a really powerful wizard...