Halfords do a single pocket think was £16.99 (in stock over at the arnison centre) - you'll also need the ISO adaptor at £12.99 if memory serves me correctly and then the single din head unit.
Alternatively there are double din - but if the car is a classic, think they are single din anyway as standard ...aren't they?
If it is not a subaru head unit you will still need the ISO adaptor (ebay is much cheaper though)