where is the best place to get this, just thinking ahead really? and how much does it normally cost?
1 day insurance is quite steep - try Richard Eggar or CCI (sorry don’t know numbers, but they will be on Seloc Wiki).
Best bet is to arrange it at your next Ins renewal - provided you can meet the specialist’s criteria (age/intended use/limited mileage etc) as they can include trackday cover. Can highly recommend Matt Taylor at Henderson Taylor 0161 437 7442, & the aforementioned brokers will also help.
I am insured with competition car insurance and I can honestly say they are awesome to deal with. My premium is low and I get 5 trackday days cover free with the policy. You can add more if you like.
Do they cover Spa?
Its just uk tracks organised by the big track day companies. Not sure if they will do a one off for spa. Havent asked. They definately dont do the 'ring.
I doubt anybody does the ring, Richard Eggar who I am with currently won’t add Spa as a free day and wanted �450 per day last year for the bookatrack days 
I went to Spa last year without trackday insurance in my 2 week old 300hp honda car. And it rained, a lot … I nearly stuffed it twice on the first lap. Great fun !
I spent a few hours yesterday ringing round which was the biggest waste of time ever. I’m with CCI (�340) and get the 5 days plus additional days at �5 per �1K insured, so �90 a day for me. They will cover one European day, not sure if they mentioned Spa but worth a ring.
Also, they did mention that if I was doing something like the SELOC French Frolic they’d be flexible on additional cover.
If it’s just a one off day then http://www.moris.co.uk/ came out at about �85 for me.