I can only share with you my personal experience. Literally, every reseller I know will help their customers even if the firewalls they have are end of support or software they use is not officially supported by the vendor. The biggest issue is that if you need vendor's support at any point, it will most likely, be very minimal. Some vendors may offer you "best effort" support, which can be described many ways, depending who you ask. As guys said here, your best bet is to contact your local SE and discuss your options.
Yes, firewalls will continue to work, but you are taking a huge risk, because if something was to happen down the road, chances of getting an RMA or support are very low.