Finding a good used shock
Aside from a couple options, there is no decent stock shock from another bike that will bolt onto the EX250. There are a multitude of problems, the major one being that the majority of them are oversprung for our application. Also, the oil in them should be changed every five years, and the older ones are not readily accessible for this service.
In addition, most of the newer shocks have a piggyback reservoir design (the oil reservoir is attached to the top of the shock). This makes it difficult to fit into the limited room of the EX250. It's not impossible, but you shouldn't consider a GSXR or similar piggyback shock mod unless you have a LOT of time or free access to a machine shop to do the requisite grinding and cutting to make it fit. You're much better off using the time to find a shock with a remote reservoir.
How to buy a used aftermarket shock
So, where does this leave us? In the aftermarket, that's where. It may not be the easiest thing to do, but finding a used Fox shock and rebuilding it yourself should be the most cost-efficient way to go. Fox tends to top the list as it's a better shock than the Ohlins (yes, seriously) and cheaper than any Penske. It's also very easy to self service. Also take a look at the available aftermarket shocks. Most can be special-ordered, so you can get the correct spring rate for your weight.
A Fox from earlier-generation (1995 and earlier) GSXR's will fit nicely, and only a very few Fox shocks have a piggyback reservoir; most are remote. The nice thing with those shocks is that if the spring rate is off by one, they're easy to change and need no revalving. So, if you found one with a 475, and wanted a 500, you'd just need to buy a 500 in/lb spring and get someone to install it.
Fox shocks were pretty common in the past. Search for a Fox from any Suzuki bike from the 1990~2000 decade (GSXR, Bandit). Chances are they'll fit perfectly; you may have to get the seller to measure the length for you. They should also be very close in spring rate. When searching, also look for a Fox from an EX250 or EX500.
Spring rate guidelines (rider only, no gear)