は with ある

こんばんは！Could someone explain to me when it would be appropriate to use は with ある? My textbooks repeatedly keep hammering the point that you're supposed to use が with ある. However, having exchanged a couple of emails with my Japanese friend, I've noticed that she'd used は with ある a couple of times. Don't think I can unearth those emails now, though.

I think you misunderstood something. ある is not special with regards to は・が. There are tons of situations where は is used.
(examples は あるの英訳｜英辞郎 on the WEB：アルク

The ways は and が are used aren't really different with ある than with any other verb.

Maybe you're just thinking of certain expressions often taught early, ie,
もっと大きなシャツがありますか
すしを食べたことがありますか

But that doesn't make it が for every sentence where ある is the verb. I can't imagine any textbook would make that claim.

Millions of words have already been written on は vs. が though, and I'm not in any way qualified to explain that endless puzzle.

The following thread/post, especially Glenn-san's explanation would be helpful to understand the difference between は and が. (Although I agree with a statement that は/が problerm is one of the most tough-to-grasp things for non-native learners.)

は, が and を what's the difference between the following sentences? | Japan Forum
Difference between arimasu and desu | Japan Forum

In your example, は seems to perform as a contrastive marker. Anyway, it's not uncommon to use は with ある, as Chris-san wrote.