if you can buy a sr5 preamp depends on the situation.
ernie ball does not sell loose preamps, only exchange old for new.
if your old one is out of warranty you will have to pay, if its still under warranty you wont have to pay.
so, if you have a broken preamp and need a new one contact the distributor for musicman in your country. they will help you get what you need.
if you are building your own bass, or modifying some other bass there are slim chances for finding a sr5 preamp.
you can scout ebay and craigslist and sometimes you might get lucky.
you probably have to get very lucky because i cant remember ever seeing a sr5 preamp on the bay
there are of course third party solutions that might give you what you need, but since this is ernieballs site it isnt good behaviour to talk about those here.
if you are looking for that you can check out talkbass or other non-corporate bass related web sites..