Stephen's earlier post
has links to two recommended HDDs, both Western Digital HDDs available at Play.com.
Just wondering though, how have you managed to run out of space? How many games have you got which have actually installed stuff on the PS3? (they don't all install). Is there nothing you can delete to make room?
If you do get a new HDD, they are very easy to fit, & doing so does not invalidate your warranty (if it is still valid). As mentioned, you would need to back up your saves first.