First off, the rituals wizards are given for free are certainly nice but it's not that they save millions of gold with them. A quick check of the Compendium revealed following average prices for the given level ranges:
Level Average Price, gp
9-11 995,68 (not including Dark Gift of the Undying)
13-15 4.782,69 (not including Lich Transformation)
I've been using level ranges rather than only the fixed levels 5, 11, 15, 21, 25 because some levels have only very little or no rituals at all (e.g. there are no level 21 rituals in the Compendium as of today).
This means that, on average, a wizard gains rituals for 322,568.47gp for free - which is roughly the equivalent of a level 22 magic item. Even if the wizard only learned the most expensive available rituals (not counting the extremely expensive rituals from the Monster Manual) I doubt it would go over the cost of a level 23 magic item.
Anyway, how to make your warlock a better ritual caster. Let me say that there is no way - at least none that I know of - to compensate the free rituals a wizard gets. There are individual feats that let you automatically master a small selection of Rituals (mostly the "Mark of X" feats from the Eberron Player's Guide), but nothing to the extent of the wizard's class feature.
The most obvious solution would be to pursue the Vistani Heritage[DDI] line of feats from Dragon Magazine 380:
Their primary benefit is that once per day you can ignore the component cost of a mastered ritual of your level or lower from the feat's associated category. This might allow you to save lots of money on ritual components, especially if you use it to reduce the costs of rituals you cast very often (like some restoration or travel rituals).
Another and very similar option is the Ritual Mastery feat line from Dragon Magazine 405 (alas they are not yet in the Compendium so I can't link to them):
- Binding Mastery (Binding)
- Creation Mastery (Enhchant Magic Item)
- Deception Mastery (Deception)
- Divination Mastery (Divination)
- Exploration Mastery (Exploration)
- Restoration Mastery (Restoration)
- Scrying Mastery (Scrying)
- Travel Mastery (Travel)
- Warding Mastery (Warding)
These feats generally provide the same benefit as the various Vistani feats - ignore the componenet costs of associated mastered rituals once per day (although there are a few feats that do something else). In addition to that, they often provide an additional benefit like a bonus to some related skill checks, rerolls, the ability to master rituals slightly above your level or reduction in casting time.
Besides feats you can try to pump the related skills to get more out of the rituals whose results depend on a skill check. Skill Focus, background benefits, certain magic items (e.g. Runic Armor[DDI]) can help with getting more benefit out of a single casting of a ritual.
Alas I haven't found any themes or paragon paths that provide direct ritual related benefits and are accessible for a main class warlock. However, there are a few epic destinies that provide an advantage:
- Level 30 feature: twice per day use any ritual you have mastered and which does not directly affect an enemy as a Standard action
- Master of Momenets[DDI]:
- Requires psionic multiclassing
- Level 21 feature: can master travel rituals, gain +5 bonus to any skill check made while performing travel ritual, once per day cast travel ritual without component cost
- Level 24 feature: can master travel rituals regardless of level
- Level 24 feature: roll twice for all ritual related skill checks and use either result, can double casting time of non-creation ritual to half component cost
Other than that I don't know what else to say. Perhaps you could provide some more information in what direction you want to develop your warlock? Do you want to cover a wide range of different rituals or do you want to truely master a specific category?