I would assume any GC or music store could order them. Stew Mac? I got some from Suhr. Using 13s in standard tuning. was able to just use 3 springs. Love them.
that's a bit odd that 5 won't work with those strings and that tuning. i've set up many strats with 11-49 tuned to E and 10-52 tuned to E also with 3 springs and it holds just fine even when doing big bends on the G string.
from what i've found, the highest tension springs are the original floyd ones. you can tell them from the typical strat springs since they have a bit of a greenish goldish hue to them and aren't pure silver.
I ran into the same issue with the same tuning with 11-49 strings on my RG3120. I ended up selling it and got a USA Robin Tedley. Set it up with the same tuning/gauge with 3 springs no problem. Weird.