I remember having to call them (Tamiya USA). If they have the kit, they will mail you the entire sprue. You will need a credit card because you will need to buy it.
https://www.tamiyausa.com/
I've also purchased sprues from this eBay seller. If you contact him, he may have the sprue you need. He has the "B" sprue listed for sale. Or you can click on the seller store and then spare parts sprues and search for yourself.
https://www.ebay.com/itm/Tamiya-1-48th-Scale-Supermarine-Spitfire-Mk-I-Parts-Tree-B-from-Kit-No-61119/293634919900?epid=23026050757&hash=item445e014ddc:g:micAAOSw23tdxwfe