welcome to GoY Bob1953.

have you tried googling plant suppliers in this country first. failing that see what usa supplies say about posting to the uk. They should be able to tell you via their websites. good luck.

Bringing plants into the UK from the US is very problematic and expensive for individuals, and not worth the hassle even for the very few species that just might be allowed, and I doubt Acers are one of them.
Also not sure you have the correct plant name , I have heard of Acer palmatum var. heptalobum, and Acer saccharinum 'Lutescens', but not Acer heptolobum 'Lutescens'. You can source nurseries listing both the above in the RHS Plantfinder.

I would certainly think that it would be fraught with difficultes from the authorities regarding health certificates (I can't spell the correct word). For one thing, I doubt that it would be allowed to have any soil around its roots. Try searching for an H.M. Governement Customs and Excise site to see if that gives any advice.

Hi Bob, Mr MB is referring to a phytosanitary certificate. However, like Bluespruce, I really doubt you could get one. Much easier to buy from a UK supplier and, I would have thought, cheaper too.