Hopefully this may be of use to others who own this bandsaw, or clones of it, and are trying to source a belt quickly.

I have so far found that Naismith Engineering have the belt in stock (actually, it gets cut from a sleeve of stock, so you can specify how many ribs you want, which may be useful because my Jet 1410 lathe uses the same length belt, but with only 3 ribs)

https://www.naismith.com.au/

(No financial or other ties to this business, apart from having just ordered belts from them, and receiving a confirmation that the order will be posted tomorrow.)

I could not find any auto parts supplier who carries this belt - it's a "J profile" which is finer than the "K profile" belts typically used in cars.

Other unsuccessful ideas were bearing suppliers, woodworking machinery suppliers, and engineering suppliers, and ebay (some UK and USA suppliers did have them, but the shipping costs were typically around $100!)

FYI, other info collected during my search:

The belt specifications are:
240J4 Polyv-rib belt.
240J4 Poly-V Belt, 24" Length (610mm).
4-PJ610 Metric 4 rib Belt.

Replaces:
Goodyear Engineered Products Poly-V Belt# 240J-4.
Metric Code: 4PJ610
Gates 240J4, Gates Product: 917102404.
Band saw drive belt. SEARS PART NUMBER: 1-JL20020002. Replaces obsolete part 29502.00.
RIKON 10-325 DRIVE BELT (Craftsman) 1JL20020002.
Black Rubber (Neoprene) belt can be used in place of Grizzly PH8259026 belt on Mini Wood lathes.
Belt for Harbor Freight/Central Machinery 10x18 Mini Lathe 65345, Grizzly H8259, et al.
Replaces Rikon C10-995 Multi V-Belt For 10-320, 10-321, 10-325. (1-JL20020002).

Cheers,
Andrew