I got the full StrongStrut kit - strut brace & firewall braces - direct from the US. This was 3 years ago and, at the time, including shipping & the VAT added by customs, the full kit came in at just under £500 (but the exchange rate has a big effect on this)
Installation thread and initial impressions is at https://z4-forum.com/forum/viewtopic.php?f=2&t=76796
There are other alternatives, including the OEM brace, and they should all do a similar job, but I liked the aesthetics of the Strong Strut