most providers would prefer to use their own. There are several reasons for this.  In any case, this is something you would need to know ahead of time.  Some may allow you to byo condoms, but they should be sealed in the box they came in, to minimize the chance of tampering.

Swim

because every so often we hear of a provider who forgot to bring hers.

A question for you providers--do you mind if the guy brings his own condoms to the date? I like a certain brand and size, so I would prefer to use mine, as opposed to what the lady has. Thanks for your response.

So, I have not had to worry about it.  I do not carry condoms with me to dates.  I think if one sticks to quality providers, the chances of this happening are greatly reduced.  Those ladies do not want something like that on their reputation, IMHO.

Swim

You can bring your own, but be prepared to not use them. Some ladies just only use their brand, others are allergic/sensitive to certain brands, some just don't trust anything that they haven't purchased themselves.

a provider who wasn't glad to use mine. And I don't keep them in a sealed box, which would be kind of impractical, since I buy boxes of 12. Maybe I just appear trustworthy. lol

unless you have a something that we try and I just don't like (rare) ....then I'll switch to mine.  Many providers will bring different kinds and sizes.

It is good protection from certain misunderstandings & oversights, a few scams, & inconveniences.  I have since once a Favorite lady called me back enroute & asked me to stop & get some (offered to reimburse, which of course I did not accept).  However, since I was on the road, in a unfamiliar area I wasted time that might have been critical, and had to settle for a brand that was not my first choice.  Since then, I always have at least 3 with me.  If a new aquaintance, I bring a sealed 3 pack...  leave the excess.  If a repeat meating, I'll bring 3 from my big box.  (Current, in date, properly stored.)
Once a lady said "I have them" when I placed them on the bedside table, before retiring to her bathroom.  I said simply "I like this kind"...  

I respect a lady's right to only use products that she provides, if she so chooses.  I've never had it happen.  
Why 3?  If one should get spoiled in application or mis action need to be replaced...  we're golden.  The fact she has 3 to choose from, puts her in control again.    
skb

I like it when a guy brings his own if he has a preference or needs more (or less) than the average size rain coat.