There are a few places you can get them from. In the past I've bought the 14 day ticket direct from Disney (disneyworld.co.uk) and I've bought them as part of a package holiday (Virgin Holidays in my case).
You can, I believe, also buy them from the various DisneyStores in towns around the UK although I've never tried.
Finally, there are a few other places on the internet that will sell them but if you do this, do your research first to check they are genuine. If they're significantly cheaper than the 'official' disney price then I'd be suspicious.
If you buy direct from Disney then it's worth looking at going through Quidco (a cashback payment site). You'll only get 2% of the money back and it make take a bit of time, but for 2 adults and 2 children, you'll save about £20 (every bit helps right!)
As for seaworld, I think buying it direct would be the way to go. I don't know if it's any cheaper to buy it in advance or not but you can easily buy these on the day. If you're staying offsite (i.e. not on Disney property) then it might be worth looking at the tourist information booths on International Drive in Orlando, they may have discount tickets but that's just a guess.
Universal, again you can do the above and just buy on the day or buy direct.
It's worth remembering that Universal is 2 parks - one is more of a 'studios' park, (
http://www.universalorlando.com/Theme-Parks/Universal-Studios-Florida.aspx) the other is Islands of Adventure which in my opinion, is absolutely excellent and contains the new Harry Potter World section (not that I've seen it yet!) (
http://www.universalorlando.com/Theme-Parks/Islands-of-Adventure.aspx)
Take a look at the two universal parks and see which one you would rather see - prices can be found here: (including tickets that will get you into both parks)
http://www.universalorlando.com/Theme-Park-Tickets/General-Admission.aspx
The last thing to consider is the Orlando Flex Ticket (I think this is the official site:
http://www.orlandoflexticket.co.uk/pages/buy-tickets). I did this on my fist trip and it works out at great value, but it depends on how many non-disney days you do. Again, if you're staying off site you may want to swap to a 7 day disney ticket, do all of them in a week and spend the second week with the flex ticket (although Id suggest doing it in reverse order, once you've seen Disney the other parks don't always match up).
Hope that helps!
