I assume the above would be something like

Cá bhfuil ...... féidir liom stampaí a cheannaigh?

What am I missing?

Cén áit ar féidir liom stampaí a cheannach?

'Cá bhfuil?' = 'Where is?' You can use 'cá' instead of 'cén áit' - in which case 'cá' + 'ar' becomes 'cár':

Cár féidir liom stampaí a cheannach?

'Cá háit?' is also used for 'Where?', and of course, there are other ways to say 'can'.

Why "bhfuil"? Bhfuil is a form of the verb bí.

"Where" is simply cá (+ indirect relative clause)
"I can" is (usually) "is féidir liom"

So, just put both together:

Cá + ar féidir liom = Where can I ... ("ar" present indirect relative form of "is")
Cá +ar is merged into cár:

Cár féidir liom stampaí a cheannach?

Or you could use another form for "where":

Cén áit (cá háit) ar féidir liom stampaí a cheannach?

There are other phrases for "can" which consist of a form of the verb "bí".
But they rather mean "to be able to". So they don't make sense with "where":

Cá bhfuil mé in ann (ábalta, in acmhainn) stampaí a cheannach? "Where am I able to buy stamps?"

Or: Cá háit a dtig liom stampaí a cheannacht?

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Thanks all. Didn't know that "ar" was present indirect relative form of "is".

I assume that "ar" in this form can come in very handy in many places.



PS:

Just to make sure I have this,

"Where can I buy some milk?"

would be

"Cár féidir liom roinnt bainne a cheannach?"

Am I correct?

We add the "t" in Conamara too.


And the reply will probably be: "Sa bposta"