I'm fairly confident this wont go down well with the other members...
Are you sure about that?
Are you essential to the future success of the business, and if so, is there potential to retain some level of involvement, even if you do sell your shares - perhaps on a 3 day week, or long enough to train someone in that can replace you.
Your fellow owners are the people likely to pay you the best price, for your stake in the business.
Selling a minority interest in a privately owned company won't be easy, as few strangers will want to buy in as a minority shareholder.
Assuming the business continues to be profitable, so has funds available for distribution, the company can buy back your shares, and cancel them (subject to complying with company law).
If I were you, I'd ask the other shareholders to consider buying you out, without discussing any particular figure, and see what they say.
Consider a degree of flexibility when speaking with them, perhaps agreeing to sell down your shares over a few years, if they can't raise sufficient funds to buy you out on Day 1 (and you have one flexibility on when you need all of the cash).
If your fellow shareholders don't want to buy you out, a key supplier, or large customer, may have an interest.
Getting on to professional advisors, I don't think you need a Big 10 firm, and I definitely wouldn't go near a Big 4 firm, for the reasons outlined above. That's not to take away from any of the firms that have been mentioned - they are all good, but they will also be quite expensive.
There are several accountancy firms, that have strong well regarded teams to help advise, structure and negotiate buyouts, some of which are outside the Big 10. Names that spring to mind outside the big firms, and should be a bit cheaper, include Cronin and Co., JPA Brenson Lawlor or PKF.
I'd suggest that you also do a little research on how to value your business, if you haven't done this already - there will be different methods, such as Net Book Value, Enterprise Value, Break Up Value etc. Then try to form a reasonable view on what you think you'd like for your shares - keeping in mind that a minority 40% stake rarely sells for its full value.