A big batch of Vredestein Classic Sprints in 135/80 14 size went over from NL to the UK a couple of years ago. Vredestein produced a batch subject to a minimum order of 1000 tyres for the DAF Club Nederland. I don't know how big a batch was ordered, but I heard 500 went over to the UK. The size hadn't been available for years.
Vintage Tyre Supplies did have them for a time, but they are now out of stock again.
The 145/80 14 size is the next option. Bridgestone make batches from time to time in Japan and these were originally made for the economy Audi A2 / VW Lupo 3L as low rolling resistance tyres. VTS are out of stock of these.
Camac (in Portugal I think) do a 145/80 14 size, that was cheap, but then it went AWOL for a while, VTS appear to have stock of these.
www.vintagetyres.com/item/141/145sr14-camac-bn313--car-tyre--road--tlAny other tyres in 145/80. 14, such as the Kumho, Firestone and any other 135/80 14 tyres, such as the Michelin X, will be rather old stock and well past it. Like this:
www.ebay.co.uk/itm/261268010584 I wouldn't touch with a barge pole as it'll have been made in nineteen canteen. Fine for a museum exhibit, but dried out and cack for a road vehicle.
I prefer a skinny tyre myself, and bought a set of 5 Vredesteins from the DCN a while back. Keeps the steering nice and light. (And my pockets rather empty.)
So at the present moment in time, you choice is limited to Camac in the UK.
After that, I'd rather bag a set of 4.5J x 13 rims as fitted to late 66s. The choice then of 155/80 13 tyres is rather bigger (and cheap, cheap).
EDIT:
Here's a nice stack of Bridgestone B391s:
p4300001 by
macplaxton, on Flickr
(bought April 2008 from Bull Motif - Morris Minor / Austin A30/A35 specialists)
Here's a nice set of Vredestein Classic Sprints:
DSCF3496.JPG by
macplaxton, on Flickr
DSCF3498.JPG by
macplaxton, on Flickr
You can save yourself a lot of hassle shopping for tyres by doing this:
a) ask a local supplier if they can get
b) and if they cannot, ask how much to fit if you can get.
I did come across one grouch who didn't want to fit "internet bought" tyres. I said I understood and would happily buy if he could supply, but as he couldn't supply, he said he would be happy to fit if I could source them.
Vintage Tyre Supplies:
www.vintagetyres.com/Longstone Tyres:
www.longstonetyres.co.uk/There was some other fella that sold tyres, I can't remember his name right now, but he handily supplied me with some German 5.20 x 13 cross-plies. A set of 4 Heidenau P36 tyres was a good bit cheaper than some Dunlop C49s for my Regal.
www.reifenwerk-heidenau.de/?lang=en (Years ago I bought a pair of Dunlop C49s and they were made in Zimbabwe at the time (1991))
And if you think some of those prices are high, they aren't really, well not compared to metric Michelin TRXs on some Citroen CX alloys!
Oh and another thing. Some tyre fitters get a nose-bleed when asked to fit a tubeless tyre to an old rim and insist on fitting a tube. Having stripped a set of DAF wheels before fitted with tubes, I found it a bad idea as there was lots of corrosion on the wheels around the valve holes, one even ended up keyhole shaped! This doesn't happen with tubeless fitted ones. The reason the tyre fitter breaks out in a 'elf and safety sweat is that old rims don't have the anti-roll-off ridge and so if they suffer a blowout then they can potentially roll off the rim. Tubed tyres are a pain in the town halls though, as when you get a nail in the tyre they go flat instead of sealing themself to a degree. Horses for courses, I rather have them without. If it was good enough in the 60s/70s then it'll do me.
Another Edit to add:
A couple of my previous ramblings on the subject:
essexdafs.proboards.com/index.cgi?action=display&board=techassist&thread=288&page=1#4095essexdafs.proboards.com/index.cgi?action=display&board=general&thread=329&page=2#4958
