Most good shops will have a full time year round boot fitter but summer is hit or miss when it comes to buying boots. Most shops won't have the new stock in quite yet so you will be fitted from remaining models. Now if the guy fitting you is reputable he should be honest and say if they have anything that will fit, but if he just wants a sale while its quiet he may just stick you in something that sort of fits.
The very best time is the very start of the season. That is when there will be the most selection so you will get the bet possible boot for you. Yes it won't be on sale but getting the right boot is so inportant. Also things like footbeds never go on sale so you will not save on those.
Overall maybe go chat with some fitters, if they happen to have a boot in stock which is perfect for you then getting it cheap is always a bonus, just don't buy something that only sort of fits because its cheap.