Getting a spell as an at will SLA

[Crake]

Is there any way that, via sacrificing spell slots or the like, you can get a spell as an at will SLA?

The spell in mind is quite low, and the method can be almost anything really, barring permanent form changes please.

If it matters, the spell in question is detect thoughts, and I'd prefer it not be via an item, or, if it is via an item, have the DC scale with the user's ability score and/or feats/abilities that raise DCs

[Firest Kathon]

if it is via an item, have the DC scale with the user's ability score and/or feats/abilities that raise DCs

Staves and scrolls use your ability score and feats to determine DCs, with staves also taking into regard your caster level (if it's higher than the staffs caster level).

[JDL]

Archmage (http://www.d20srd.org/srd/prestigeClasses/archmage.htm)

High Arcana: Spell Like Ability

This will only give you 2/day uses and costs a 5th level spell slot as well as the spell's slot too.

An easier way is the Helm of Telepathy (27,000 gp, DMG) for at will detect thoughts plus other goodies.

[Karnith]

The Complete Arcane version of Innate Spell (which has been superseded by the PGtF version, I think) allows you to get a spell as an at-will SLA by sacrificing a spell slot eight levels higher than said spell's level.

It's not really doable for Detect Thoughts pre-epic (barring some serious cheese), though.

[Jack_Simth]

There's also the Innate Spell feat from Complete Arcane. You will, however, need three other feats and a spell slot eight levels above the one you want to make Innate. As Detect Thoughts is a 2nd level spell, you'll need a 10th level spell slot to burn. This is doable before 20th. A human Wizard-5/Red Wizard (of Thay)-10/Wizard+3 - Red Wizard lets you Heighten spells as high as 20th with a little help, Extra Slot (Complete Arcane) gives you an extra spell slot up to 1 level lower than the highest-level spell you can cast, and then Innate Spell gives it to you.

[Bronk]

It looks like the only way to get this ability as an at will SLA is to take four levels of Exorcist of the Silver Flame.

http://dndtools.eu/classes/exorcist-of-the-silver-flame/

[doomlord]

It looks like the only way to get this ability as an at will SLA is to take four levels of Exorcist of the Silver Flame.

http://dndtools.eu/classes/exorcist-of-the-silver-flame/

Or one of the many other prestige classes that give Detect Thoughts at will. Anything from Cabinet Trickster 5 to Rainbow Servant 10 to Void Disciple 13. These are all capstone abilities, though. (Cabinet Trickster is a five level class, Rainbow Servant is a ten level class, and Void Disciple is a thirteen level class. Although do note that the Cabinet Trickster gets to use it 2/day at level 1 and 4/day at level 3.)

Insidious Corruptor gets "probe thoughts" at will at level 2, which functions like detect thoughts but confuses the target as well. It does take two standard actions to use it though, as you first need to put them under your "Claws of Influence". (And it's a pretty horrible class, all told, so there isn't much point.)

Although, in any case taking Rainbow Servant is quite good. It's full casting, and it adds quite the selection of spells to your class spell list, up to the whole Cleric spell list at tenth level. (Plus other cool stuff like wings, and the ability to use detect evil/chaos/thoughts, dependent on level. And the granted powers of the Good, Air, and Law domains. It's quite the package, all in all.)

[Bronk]

Cool, nice catch!

[eastmabl]

This requires the slightest amount of homebrew.

One could propose to a DM that detect thoughts could be a least invocation for a warlock (other least invocations SLAs that are the equivalent of 2nd or 3rd level spells).

Assuming this grant by the DM, all you need is a one level dip to get the spell as an at will SLA.

[(Un)Inspired]

What about anima maging and then binding dantalion?

[Fax Celestis]

Dantalion (5th level vestige) gets a detect thoughts-like ability.

[Zetapup]

Moonstar Agent hasn't been mentioned yet. At 2nd level, it gives you detect thoughts 3/day.
The Pixie racial progression from savage species give you detect thoughts 1/day at 3rd level. Pretty meh.

Personally, I'd say that rainbow servant looks like the best option so far, although you're going to have to talk to your dm about the spellcasting progression (table says 6/10, text says 10/10, apparently this switches in some languages so the "text trumps table" rule doesn't work as well). If you're looking for classes to go into after you have at will detect thoughts, you could consider mindspy. It seems to fit the flavor decently.

[ddude987]

dragonblood sorcerer racial subsitution levels get any spell you know at level 4 as an SLA 3/day.