Anything special about these, or will any good brand of the correct size be OK?

I am afraid you will have to go for the OEM ones, Gates makes them specificly for Ducati and none of the other brands will fit properly.

There is ONLY one alternative I know of and has been developed by Chris @ California CycleWorks, they are good and cheaper that OEMs, but I am not sure they have the profile version used on the old Pantah engines.

Since you are looking for a replacement, I would source from Ebay a set of Camshaft pulleys from the newer versions of the V2 engine (models post MY2000) that have a better belt profile and are ADJUSTABLE, then you can eiyther fit the OEM belts of the California Cycleworks...

California Cycleworks do Exactfit belts for the bikes that use square profile tooth belts.
You need belt TBF1 which fits;
Ducati 750 F1 All models
Ducati 400 F3 All models
Ducati Indiana 350
Ducati Indiana 650
Ducati Monster 400 (1997 and older)
Ducati Monster 600 (1997 and older)
Ducati Monster 750 (1997 and older)
Ducati Pantah 350
Ducati Pantah 400
Ducati Pantah 500 SL
Ducati Pantah 600 SL
Ducati Pantah 650
Ducati Pantah 750
Ducati Paso 750
Ducati Supersport 750 (1997 and older)

Bimota DB1

Cagiva Elefant 350
Cagiva Elefant 650
Cagiva Elefant 750
Cagiva Alazurra 350
Cagiva Alazurra 650

They are £47.95 a pair from the UK distributor. http://www.exactuk.com/exactfit-belts-fitments-price-payment/

The 906 engine is fitted with the modern round tooth profile pulleys same as the 907, 900ss, 750 & 900 Monster and ST2.
As angelix mentioned you have 2 choices, OEM or Exact Fit. I did a quick search and got a few different prices.
Gowanloch Ducati in Sydney has the Exact Fit for $140pr, Brad Black in Oakleigh(Vic) has the same for $121 and Mornington Ducati(Vic) has OEM belts listed for $108.85

There's also Flennor.

Derek, are you sure the TBF1 are right and not the TB900? The 906 belts are the same as the ones of the 907IE.

angelix, "I would source from Ebay a set of Camshaft pulleys from the newer versions of the V2 engine (models post MY2000) that have a better belt profile" What's different in the profile? Some belt fitment lists, list the 906 but also the ST2 -2003 or the MH.
If the profile was different and the cam pulleys would have to be swapped, the driving pulleys would have to be changed as well.

Nah! I picked up on Angelix's query about the profile for old Pantah engines

The 906 of course uses the same round tooth belts as the 900SS etc. as stated by Duc906 so needs the TB900.
Good to know you can get square toothed belts though

The brand Flennor has been mentioned.
Do others agree this a safe choice?
Someone in Germany has service kits that include them, as well as filters etc.
It would be convenient to get the complete kit.

I replaced mine by myself with a pair of Dayco, 10k km ago (odometer at 60k km). In further 10k km I'll replace them with the same brand. In my opinion good and cheap in one.