Nuclear weapons cores made from Pu-239 don't really degrade in storage and the material can always be reformed into new warheads as demands require but only in expensive facilities. The equipment around the cores does degrade -- for example the chemical explosive lenses arranged around the cores are precision devices and close proximity to a large amount of radiation will degrade them over time as will simple ageing and they do need to be swapped out as necessary. It's part of the expensive business of owning deployable nuclear weapons.
Britain has something like 70 tonnes of weapons-grade Pu-239 surplus to requirements from the time we had nearly a thousand warheads (we now have less than 200) and the US and Russians have a lot more than that in storage. It's just sitting there in several expensive storage facilities "somewhere". There's a much bigger cost penalty to converting material like that into usable weapons, not including the missiles, submarines and aircraft required to deliver them, the personnel to operate them, the training, security, release protocols etc. so most of those decommissioned warheads from the 60s and 70s have been dismantled beyond the point where they could be quickly put back into service. The US maintains a second-string reserve of warheads, mothballed at great expense beyond its incredibly expensive front-line fleet ready for use sitting on top of Minuteman-IIIs in South Dakota or riding in Ohio boomers somewhere in the Pacific. Notice the use of that word, "expensive". You tend to see it turn up a lot in discussions about nuclear weapons and materials. Keeping a secret stash of nukes and/or Pu-239 costs a lot of money, it can't simply be parked in a shed on an army base somewhere with a padlock on the door.
The Russians have sold the US a lot of highly-enriched U-235, some of it from weapons cores which has been downblended into nuclear fuel for power reactors, the "Megatonnes to Megawatts" project. The Russians surplus Pu-239 stockpile is more of a challenge but they are looking at using it in their BN-series fast reactors as well as MOX fuel for PWRs and the like. The US still hasn't licenced any MOX operations in its own commercial reactor fleet although there's a good deal of operational experience with it elsewhere in the world. This is the obvious way to use up surplus weapons-grade Pu-239 but the security of moving such materials around to downblend it is problematic -- commercial MOX with pure Pu-239 is a very great security risk.
U-233 -- no, the US does not have hundreds of tonnes of the stuff. It has two tonnes, no more. There are no real thorium reactors planned, granted Construction and Operating Licences (COLs) or pouring concrete now or in the forseeable future (unlike the US where there are four new-generation PWRs under construction and several more COLs have been issued). Assuming a series of financial, regulatory and licencing miracles occurs the earliest a molten-salt thorium reactor would be starting up anywhere would be fifteen to twenty years from now, and even that's optimistic as long as gas is cheap, coal is cheaper and yellowcake is $35/lb at the minehead. Storing that bomb-grade U-233 is expensive (oh look, there's that word again!) and it's not necessary to use U-233 to start up a thorium breeder, this can be done using U-235 and Pu-239 as the Indians plan to in their heavy-water PWRs in a thorium/MOX fuel cycle.