1. To start:
Among the best are
Fergusons. Purportedly the last weavers of linen damask in Ireland.
A range of weights, yard goods, hemstitched, etc.
Embroidered, appliquéed, etc.
If you google "pure linen napkin," "French linen napkin," "Italian linen napkin," you'll find many, many more sources, including high-end design ranges, more rustic styles, etc.
2. When you say replacement, do you mean that you want some new napkins that will be used in conjunction with some originals having maiden-initial monograms, or a complete set of new ones? If the former, it would also be useful to know what format the maiden-initial marks take, and what the couple's attitudes toward traditions are, including whether the wife retains her maiden name in professional life, etc. I believe the oldest tradition on this topic is that young women embroidered their first and maiden initials on their trousseau linens with space left in the center where the married-surname initial would be added when they became engaged. At least in the U.S., once it became uncommon for young women to embroider their own linens, bridal trousseaus were traditionally prepared after the engagement and marked with first, married, and maiden initials in that order, for use after the wedding. In recent generations, the bride's pre-marriage linens have often been marked with first and maiden (or first, middle, and maiden) initials with no space left for the married one, which sounds like the case you are describing. It is also not uncommon for table linens purchased post-engagement for a married couple to be marked with the wife's first initial, the married surname initial, and the husband's first initial, in that order.
The traditional etiquette seems to be, as most etiquette should be, based on practical considerations that conduce to social ease. So marks on items that might be expected to pass to a younger generation, such as silver, and perhaps pure linen table linens not intended for daily use, traditionally always included (or were limited to) the married initial, so that the family name continued to be represented. Likewise, classically, a married woman continued to use her engraved stationery with maiden initials, since the cost of the engraver's die was substantial. This custom has relaxed and may now be a minority practice where the married woman has adopted her husband's surname, since true engraved stationery is less common.
So if you are replacing an entire set of napkins, you can choose a rationale from the above (should the wife maintain her own surname, you'll have a delicate choice as to which partner "represents" the home and its domestic furnishings). If you are only replacing a few napkins that will be used with several originals, you can either maintain maiden-initial consistency or choose one of the married-initial variants above on the theory that eventually all the napkins will be replaced--resulting in a mixed table in the interim. The no-initial option, if chosen, would be based on the same logic.
For new purchases, my own preference is not to use monograms, but since heirlooms may well include monograms, ciphers, or crests from both sides of a family (or indeed, for collectors, pieces of fine quality with unrelated family marks) there can be great charm in a mixed table, and this is quite often seen now. (If you are fortunate enough to have several complete sets of linen or silver with different marks, so that each can be used without mixing, you are fortunate indeed.) I hope this is of some use in thinking through the issue. If you are planning this purchase for your own household, you might review these options with your spouse. If it is to be a gift, perhaps consultation would still be possible, unless it is to be a surprise.