You can fudge a filter cap quite a lot. 11,000uf is really an odd value these days, but 12,000 or even 15,000 should be readily available and will work just fine in that application.
-Ian
Just to toss in some confustion factor.
There are several "E" series for capacitor values:
E3 = tens multiples of 10, 22 and 47 (i.e. 1uF, 10uF, 220uF or 4700uF)
E6 = tens multiples of 10, 15, 22, 33, 47, 68 (standard series used for caps)
E12 = tens multiples of 10, 12, 15, 18, 22, 27, 33, 39, 47. 56, 62, 68, 75, 82, 91
E24 = much wider varietity and includes 11 (too lazy to list)
The higher "E" series are typically available but are often not standard values so they typically cost more.
E6 is the standard series used for capacitors so a standard valuein your range would be either 10,000uF or 15,000uF.
12,000uF is the typical capacitor used to replace 11,000uF caps. 12,000's fall into the E12 series - usually special orders and cost more than E6's. Axial 25V is in the $5 range.
11,000uF would fall into the E24 range. It would also be a special order but normally costs even more than the 12,000uF caps. Probably looking at a 25V axial cap that's over $7.
A 12,000uF cap is not cheap... but an 11,000uF cap would still cost you considerably more.
The cap the original poster wants is used on the Williams power supply for main regulator -- anything in the 11,000uF to 15,000uF range will work (like Ian said). 10,000 would probably work just fine as well but as a rule, I don't reduce capacitance value without fully analyzing the circuit (and most of the time, I'm just too lazy to do that).
GPE normally sells 12,000uF and 18,000uF axial capacitors for Williams power supplies but are currently sold out of 12,000uF's. More on order but since they are special orders, they have a long lead time - not due until October.
Ed