For the mid range costs (£15-30), last season for high street shopping (River Island and Marks & Spencers) had some pretty decent white shirts with decorative blue embroidery in a similar style
For cheaper (£3-15), but a bit more DIY I know that someone I know in Varushka did a thrift shop haul, found a plain white shirt with the style sleeves and neckline that they wanted and then embroidered/wove thin ribbon through the broader anglaise (sp?) holes to great effect
(Alas, I lack photos, as they wore it during E1 for the coronation stuff. Hopefully there’ll be some good shots as they were one of the folks who performed a solo at it)
There’s also commissioning folks. Empire has some amazing costumers and costs range all over the shop…but you’d probably be looking at £40-60 at the low end for a plain handmade one and upwards of £100 if you want embroidery.