Congratulations on buying a RIB! What did you get?
An A-frame and depth sounder are not what I would consider "must have" accessories, although they are handy to have. If your budget is limited then there are other things that really should have priority. You may already have everything else, but just in case here are some things I would buy first . . .
The best way to get an A-frame at a good price is design it yourself and get a local engineering company to make it for you. This will cost about a third of the retail price, but does require some work! Otherwise try talking to a couple of RIB manufacturers and see what they can offer.
There seem to be very few A-frames sold as generic accessories, but it might be worth calling John Bridger Marine in Exeter 01392 216420 who I have found very helpful in the past.
Don't know much about depth sounders -- I've never had one! If I was going to get one I would save up for a forward looking one from EchoPilot though.
Have you got a drysuit? If you haven't, I would seriously suggest that you invest in one of these instead of either the A-frame or the sounder -- it will make much more difference to your boating enjoyment!