The Lotus A-prefix HTD belt is HCR rubber, which is late 1960s technology. The B-prefix belt is HSN rubber (Highly Saturated Nitrile), is 1995 technology, and was the last "Lotus" belt for the 9XX 4-cylinder engines.
HNBR is the current technology, and it's different from HSN... they're not the same.
I'm not aware of any mainstream manufacturer making an HNBR version of the round tooth HTD belt in a size to fit the 9XX; with the exception of Gates Racing, which is a low volume specialty division of Gates Rubber. JAE commissioned them to make the blue HNBR HTD belt for Lotus engines (4-cyl & V8), so Gates makes it exclusively for them. It doesn't appear in Gates' catalog, and isn't available from Gates retailers. JAE does sell to other Lotus specialists, including Lotus Cars, plc, so it's available from more suppliers than just JAE. It's expensive, but it's good.
Gates Racing does make a blue HNBR version of the trapezoidal tooth belt... Gates P/n T104RB. Any Gates retailer can order it for you, but it's unlikely they will have one in stock.
Besides improved reliability and durability, the Gates blue belts offer greater stability. Continue to check the belt's tension on a regular basis; however, once it's properly tensioned during installation, it's very rare that blue HNBR belt will ever need to be re-tensioned.
Blue is a Gates Racing trademark, and it's just an added cosmetic laminate. The black part is the 'working' HNBR portion, and the blue part is just marketing bling.
Not all Round Tooth timing belts are HTD, and the Gates T249 is Curvilinear I, not an HTD belt. The round tooth evolved through a series of revisions. HTD was the first, followed by Curvilinear I & II, GT, etc. Those follow-on belts will not properly fit the Lotus HTD pulleys.
The 1991 Peugeot 405 Mi, 1.9L DOHC engine uses an HTD belt with the same number of teeth as the 9XX, but slightly narrower (25.4 Vs 26.4). It will fit and work just fine, but like the Lotus belts, I'm not aware of any company making a modern HNBR version of the 405 belt. Gates does make belts to fit the 405, but they're not HNBR. Gates' belt for the 405 has it's own part number, but the difference is only timing marks that are printed on the Peugeot belt. Ignore the 'Peugeot' markings, and it's the same as the generic Gates belt of the same material.
Belts for the Nissan VG30E SOHC V6 engines also went through a series of round-tooth evolutions. Of the 3 or 4 different round tooth belts used, only the HTD belt used after August 1994 (ie, Sept-onward) fits the Lotus engine. It's available in HCR & HSN rubber, but not the current HNBR.