All deepnds on what type of wetsuit you want - if its a generic surfing one - all the shops are good for it. If its more for diving - then Great Outdoors would be the expert.
The Great Outdoors has a seperate shop for wetsuits, etc. With your back to Grafton Street, walk past the main shop on Chatham Street, turn left to Clarendon Row, then right down a small lane called Clarendon Market.
I have one of the shortie wetsuits from Lidl I use occasionally in the summer - it was €30 and is perfect for what it is - I also got a summer full length Ripcurl wetsuit in Portwest for €100.
It really is horses for courses. Also remember to check where the zip is on the wetsuit for the type of sport youre doing - for surfing you want a zip up the back - not all wetsuits are designed that way. For Kayaking etc - a zip up the front is better.
If you are buying wetsuits for children, The Great Outdoors have two very useful services.
(1) You can try on/fit on (advantage over Lidl/Aldi) to get the right size. We found the Dunnes Stores children's ones were very baggy.
(2) If you bring back a used child's wetsuit the next summer, you can trade it in and they will give you the next size up half-price.