It's a relatively small amount, you might get a joinery shop to make you oak t&g boards. Can't think of an off the shelf supply at the moment.
Chiltern timber do oak shiplap in a range of sizes - I’m looking at that. Alternative would be a single board (225 ish) both are pricey.
Going to be tough to avoid the high price wherever you source it.
@Sheppz started
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It's a relatively small amount, you might get a joinery shop to make you oak t&g boards. Can't think of an off the shelf supply at the moment.