Tower Plus EBT2250, PSU be upgraded?

Yes, the EBT2250 power supply can be upgrade from a 460w to a 750w or 1000w.  However, since it's a Dell propriety power supply, it's rather cheaper to upgrade at the purchase configurations than doing upgrade later, after purchase.

At today pricing, upgrade to a 750w will be $50 USD and upgrade to a 1000w will be $100 USD.  To upgrade in the future, a 1000w PSU will cost you over $200 USD.  

https://www.dell.com/en-us/shop/desktop-computers/dell-tower-plus-desktop/spd/dell-ebt2250-desktop/useebt2250wcto04

If you are already looking at future upgradeability, you may want to consider another option -- a system from a builder that uses standard parts rather than a proprietary pre-built desktop.

There are plenty of options out there that do not lock you into expensive, difficult to source parts down the road.  

It takes significantly longer to ship when the package is customized so I have to go with a pre-bundled package. The one I'm looking at will come with a 460 W PSU. Can I upgrade that with a standard ATX PSU that I can get at Best Buy? Or does it have to be a specific type of PSU? Again, when I looked on the Dell website, there are no PSU's for sale as an accessory to the Dell Tower Plus model. Does this mean I can get any ATX PSU for the upgrade?

Dell hasn't used standard ATX power supplies in most models in quite some time, and it has a habit of changing the wiring on the mainboard connector regularly.  They're using a 10 pin mainboard connector now (standard ATX is 24 pin).

You'll either need an OEM power supply or a converter that adapts the standard ATX power supply output to the nonstandard mainboard connector.

How do I purchase an OEM power supply if I do not see it listed on the Dell website?

You probably cannot right now (there's essentially no need for Dell to supply them, as all of the existing systems are under warranty).  The used-parts suppliers will get them in time, as systems are decommissioned or scrapped, and it's likely Dell will have them eventually once the systems start to go out of warranty.

As I mentioned, if upgradeability is a major concern, I would rethink buying any pre-built system.  They all use at least some proprietary parts.  

There are plenty of smaller manufactures that build using off the shelf cases, power supplies, mainboards, etc. without the restricted parts supply and very often, reduced feature sets that are endemic to pre-built brand name systems.

Re:  How do I purchase an OEM power supply if I do not see it listed on the Dell website?

It's the reason I have suggested to select upgrade PSU at purchase point.  OEM power supply will be available from resellers online for $250 to $300 USD (based on market pricing of XPS 8960).  Dell Parts and Accessories won't have them available unless they became surplus from warranty reserved stocks.