I think ebay may be the one, get it roughly the right size. The Bullitt is on order so I'm planning these things out ready for when it arrives!
Also have some pine planking (old bed slats) I'm going to use for the cargo deck, and run some airline rails down each edge for lashing points.
So hyped
Check Gumtree/eBay for used ones I'd say.
If you prefer new, the go-to company is AluLock:
https://www.alulock.com/en/products/aluminum-boxes
Prices shown are public prices, trade is 60% off so I'd advise looking for a UK distributor.
Brexit has added import tax and somehow VAT (Only Boris only knows where the VAT was going before Brexit) so prices can get SPICY