Back to the original question. You don't say how complex the work is? Is it a bit of fettling, or ripping the whole panel out and fitting a pair of GTNs and and Aspen?
I needed a quote for removing an inop ADF receiver, and swapping out an old mechanical King nav/com for a Garmin 430.
I contacted four avionics companies in the UK, and one in the Netherlands who was recommended to me.
UK1) Still hasn't replied to my original enquiry
UK2) Replied after two weeks and said the work would cost more than the airframe was worth and came up with about ten barriers against continuing
UK3) Replied after three weeks saying they would pass my query on to someone else. That someone else contacted me and asked for more information, and it's now been 15 days without contact
UK4) Replied immediately - same day in fact. Apologised that he was away for a couple of days but would have a quote by the end of the week. Got quote two days later. Instant replies to all e-mails, and a very helpful dialogue. Competitive pricing.
NL) From initial e-mail to getting a quote was four working days. Again, very helpful discussion. Reasonable quote, but obviously there are additional costs involved with this option (ferrying the aircraft back and forth from Holland) - that said, it's the sort of flight I'd do for fun anyway, so really all we're talking about is a couple of lo-co airline tickets on top.
No point in naming/shaming UK1-3, but UK4 was Bournemouth Avionics at Lee-on-Solent, and NL was JP Avionics at EHMZ (Zeeland).